Regression in io.fits due to undefined variable

@mwcraig @embray - I’m running into the following failure in glue due to the recent changes in io.fits:

    def _read_next_hdu(self):
        """
            Lazily load a single HDU from the fileobj or data string the `HDUList`
            was opened from, unless no further HDUs are found.
    
            Returns True if a new HDU was loaded, or False otherwise.
            """
    
        if self._read_all:
            return False
    
        saved_compression_enabled = compressed.COMPRESSION_ENABLED
        fileobj, data, kwargs = self._file, self._data, self._open_kwargs
    
        try:
            self._in_read_next_hdu = True
    
            if ('disable_image_compression' in kwargs and
                kwargs['disable_image_compression']):
                compressed.COMPRESSION_ENABLED = False
    
            # read all HDUs
            try:
                if fileobj is not None:
                    try:
                        # Make sure we're back to the end of the last read
                        # HDU
                        if len(self) > 0:
                            last = self[len(self) - 1]
                            if last._data_offset is not None:
                                offset = last._data_offset + last._data_size
                                fileobj.seek(offset, os.SEEK_SET)
    
                        hdu = _BaseHDU.readfrom(fileobj, **kwargs)
                    except EOFError:
                        self._read_all = True
                        return False
                    except ValueError:
                        # A ValueError can occur when trying to perform I/O
                        # on a closed file
                        if fileobj.closed:
                            self._read_all = True
                            return False
                        else:
                            raise
                    except IOError:
                        if fileobj.writeonly:
                            self._read_all = True
                            return False
                        else:
                            raise
                else:
                    if not data:
                        self._read_all = True
                        return False
                    hdu = _BaseHDU.fromstring(data, **kwargs)
                    self._data = data[hdu._data_offset + hdu._data_size:]
    
                super(HDUList, self).append(hdu)
                if len(self) == 1:
                    # Check for an extension HDU and update the EXTEND
                    # keyword of the primary HDU accordingly
                    self.update_extend()
    
                hdu._new = False
                if 'checksum' in kwargs:
                    hdu._output_checksum = kwargs['checksum']
            # check in the case there is extra space after the last HDU or
            # corrupted HDU
            except (VerifyError, ValueError) as exc:
                warnings.warn(
                    'Error validating header for HDU #{} (note: Astropy '
                    'uses zero-based indexing).\n{}\n'
                    'There may be extra bytes after the last HDU or the '
                    'file is corrupted.'.format(
>                       len(hdulist), indent(str(exc))), VerifyWarning)
E                       NameError: global name 'hdulist' is not defined

../../../miniconda/envs/test/lib/python2.7/site-packages/astropy/io/fits/hdu/hdulist.py:1150: NameError
============================ pytest-warning summary ============================

[embray]

Definitely an under-tested corner case 😛

[astrofrog]

@mwcraig - yep, any file that is not a FITS file that has unicode:

In [6]: with open('test.fits', 'w') as f:
   ...:     f.write('Ceçi ne marche pas')
   ...:     

In [7]: fits.open('test.fits')
---------------------------------------------------------------------------
UnicodeDecodeError                        Traceback (most recent call last)
/Users/tom/Dropbox/Code/Astropy/astropy/astropy/io/fits/hdu/hdulist.py in _read_next_hdu(self)
   1107 
-> 1108                         hdu = _BaseHDU.readfrom(fileobj, **kwargs)
   1109                     except EOFError:

/Users/tom/Dropbox/Code/Astropy/astropy/astropy/io/fits/hdu/base.py in readfrom(cls, fileobj, checksum, ignore_missing_end, **kwargs)
    328                                      ignore_missing_end=ignore_missing_end,
--> 329                                      **kwargs)
    330 

/Users/tom/Dropbox/Code/Astropy/astropy/astropy/io/fits/hdu/base.py in _readfrom_internal(cls, data, header, checksum, ignore_missing_end, **kwargs)
    393                 header_offset = data.tell()
--> 394                 header = Header.fromfile(data, endcard=not ignore_missing_end)
    395             hdu_fileobj = data

/Users/tom/Dropbox/Code/Astropy/astropy/astropy/io/fits/header.py in fromfile(cls, fileobj, sep, endcard, padding)
    449             return cls._from_blocks(block_iter, is_binary, sep, endcard,
--> 450                                     padding)[1]
    451         finally:

/Users/tom/Dropbox/Code/Astropy/astropy/astropy/io/fits/header.py in _from_blocks(cls, block_iter, is_binary, sep, endcard, padding)
    497 
--> 498             read_blocks.append(decode_ascii(block))
    499 

/Users/tom/Dropbox/Code/Astropy/astropy/astropy/io/fits/py3compat.py in decode_ascii(s)
     29         if isinstance(s, bytes):
---> 30             return s.decode('ascii')
     31         elif (isinstance(s, numpy.ndarray) and

UnicodeDecodeError: 'ascii' codec can't decode byte 0xc3 in position 2: ordinal not in range(128)

During handling of the above exception, another exception occurred:

NameError                                 Traceback (most recent call last)
<ipython-input-7-add7824ea4ef> in <module>()
----> 1 fits.open('test.fits')

/Users/tom/Dropbox/Code/Astropy/astropy/astropy/io/fits/hdu/hdulist.py in fitsopen(name, mode, memmap, save_backup, cache, lazy_load_hdus, **kwargs)
    164 
    165     return HDUList.fromfile(name, mode, memmap, save_backup, cache,
--> 166                             lazy_load_hdus, **kwargs)
    167 
    168 

/Users/tom/Dropbox/Code/Astropy/astropy/astropy/io/fits/hdu/hdulist.py in fromfile(cls, fileobj, mode, memmap, save_backup, cache, lazy_load_hdus, **kwargs)
    375         return cls._readfrom(fileobj=fileobj, mode=mode, memmap=memmap,
    376                              save_backup=save_backup, cache=cache,
--> 377                              lazy_load_hdus=lazy_load_hdus, **kwargs)
    378 
    379     @classmethod

/Users/tom/Dropbox/Code/Astropy/astropy/astropy/io/fits/hdu/hdulist.py in _readfrom(cls, fileobj, data, mode, memmap, save_backup, cache, lazy_load_hdus, **kwargs)
   1011 
   1012         # Make sure at least the PRIMARY HDU can be read
-> 1013         read_one = hdulist._read_next_hdu()
   1014         # If we're trying to read only and no header units were found,
   1015         # raise and exception

/Users/tom/Dropbox/Code/Astropy/astropy/astropy/io/fits/hdu/hdulist.py in _read_next_hdu(self)
   1148                     'There may be extra bytes after the last HDU or the '
   1149                     'file is corrupted.'.format(
-> 1150                         len(hdulist), indent(str(exc))), VerifyWarning)
   1151                 del exc
   1152                 self._read_all = True

NameError: name 'hdulist' is not defined