Fix get_num_frames_in_video

[namtacs]

Tests have shown that there can be 2 exact same numbers in the output, separated by non-digit symbols. I think it's better to just find the first number.

[tancik]

What are you seeing as the output of ffprobe? I wan't to get a better idea about when it fails.

[namtacs]

Yes, in some cases ffprobe can give results like this: 123123\n\n123123.
For example, the AVCHD codec in MTS format does this.

[tancik]

This doesn't work for me. I ran a test where it should return 635 frames, but it only returned 6.

[namtacs]

There was an error in the code: output[0] was referencing the initial command output, which is not what we want. I changed it to number_match[0]. This means "take the regex match and return the group 0 of it (whole match)".

[tancik]

LGTM