[Bug] IndexError: index 1 is out of bounds for axis 1 with size 1

Description

When I use spleeter separate a audio file, I met this error:

Traceback (most recent call last):
  File "/usr/local/bin/spleeter", line 8, in <module>
    sys.exit(entrypoint())
  File "/usr/local/lib/python3.6/dist-packages/spleeter/__main__.py", line 54, in entrypoint
    main(sys.argv)
  File "/usr/local/lib/python3.6/dist-packages/spleeter/__main__.py", line 46, in main
    entrypoint(arguments, params)
  File "/usr/local/lib/python3.6/dist-packages/spleeter/commands/separate.py", line 45, in entrypoint
    synchronous=False
  File "/usr/local/lib/python3.6/dist-packages/spleeter/separator.py", line 191, in separate_to_file
    sources = self.separate(waveform, audio_descriptor)
  File "/usr/local/lib/python3.6/dist-packages/spleeter/separator.py", line 157, in separate
    return self.separate_librosa(waveform, audio_descriptor)
  File "/usr/local/lib/python3.6/dist-packages/spleeter/separator.py", line 145, in separate_librosa
    stft = self.stft(waveform)
  File "/usr/local/lib/python3.6/dist-packages/spleeter/separator.py", line 126, in stft
    dl, dr = (data[:, :, 0].T, data[:, :, 1].T) if inverse else (data[:, 0], data[:, 1])
IndexError: index 1 is out of bounds for axis 1 with size 1

Step to reproduce

1. Installed using pip install spleeter(spleeter-1.5.0)

2. Run as spleeter separate -i 68481579482858.mp3 -o output/ (On Colab environment) Attach: 68481579482858.zip

3. Got this error

Output

Share what your terminal says when you run the script (as well as what you would expect).

Environment

OS	Ubuntu 18.04
Installation type	pip
RAM available	12GB
Hardware spec	CPU

Additional context

[mmoussallam]

Hi @DGideas

The PR has now been merged. Your issue shoud be fixed, but let us know if otherwise.

[DGideas]

@mmoussallam @alreadytaikeune I tested this file with the latest master branch code and it works! Thanks you for your PR 😃

This problem should be fixed.