Audio error handling made failsafe

[gyglim]

This PR is to improve audio reading robustness.

In case there are audio frames missing, this repeats the last frame, as is done in the case for video frames:
https://github.com/Zulko/moviepy/blob/master/moviepy/video/io/ffmpeg_reader.py#L109

Thus it makes the code more consistent and robust.
Fixing #246

Cheers,
Michael

[coveralls]

Coverage increased (+0.04%) to 46.896% when pulling 68df86c on gyglim:master into caf955c on Zulko:master.

[coveralls]

Coverage increased (+0.04%) to 46.896% when pulling 68df86c on gyglim:master into caf955c on Zulko:master.

[coveralls]

Coverage increased (+0.04%) to 46.896% when pulling 68df86c on gyglim:master into caf955c on Zulko:master.

[gyglim]

The PR now only shows a warning when not enough frames can be read (happens when there are less frames than should be, according to fps*duration). Because of that, when iterating over a larger range than the actual clip (as in https://github.com/gyglim/moviepy/blob/68df86c880ec1df0f9f3e0d4b36efa02f8c2b8eb/tests/test_issues.py#L139), it was possible to request frames completely out of the estimated valid range, while before, an error would occur earlier.
Thus, I added an explicit check that the required times are valid according to the estimated range, as otherwise the list of frames to be read is empty in https://github.com/gyglim/moviepy/blob/68df86c880ec1df0f9f3e0d4b36efa02f8c2b8eb/moviepy/audio/io/readers.py#L173

[coveralls]

Coverage increased (+0.04%) to 48.695% when pulling d3b9428 on gyglim:master into d633510 on Zulko:master.

[coveralls]

Coverage increased (+0.07%) to 48.732% when pulling d3b9428 on gyglim:master into d633510 on Zulko:master.

[gyglim]

Hey. Just added a test for this. Is there anything else needed to merge it? @Earney

[ghost]

@gyglim. Thanks for the pr
Lets see what some of the other project contributors thinks. @Zulko. Is it ok to replace the error message with this code?

[coveralls]

Coverage increased (+0.1%) to 48.79% when pulling befe989 on gyglim:master into d633510 on Zulko:master.

[gyglim]

New issues on the same:
#265
#521

@Zulko Any opposition against merging this PR? Anything I can do to improve it?

[Zulko]

Sorry i thought this was merged. Id say yes to this pr. It's a bit of a hack, but it wont be the first in moviepy, and if it solves that audio problem it's good for me.

[gyglim]

Ok, great :). I agree, but it makes sense and is consistent with what happens for video

[coveralls]

Coverage increased (+0.07%) to 48.954% when pulling 5671b17 on gyglim:master into c611107 on Zulko:master.

[ghost]

@gyglim lets merge this in.. unless someone objects?

[gyglim]

@Earney Yes, let's do it