Stream stack does not take care of garbage-collected Stream (i.e. weakref is None)

import cupy as cp
from cupy import cuda

with cuda.Stream() as s1:
    s2 = cuda.Stream()
    s2.use()
    with cuda.Stream() as s3:
        del s2

This fails when exiting s3 context:

Traceback (most recent call last):
  File "test.py", line 8, in <module>
    del s2
  File "cupy/cuda/stream.pyx", line 197, in cupy.cuda.stream.BaseStream.__exit__
  File "cupy/cuda/stream.pyx", line 34, in cupy.cuda.stream._ThreadLocal.set_current_stream_ref
AttributeError: 'NoneType' object has no attribute 'ptr'

[kmaehashi]

I think this line is problematic:

Yes, we need to rewind the stream stack until we find an alive one. If everything is dead we should fallback to the default stream.

[emcastillo]

Related https://github.com/cupy/cupy/pull/4337