ext/ffmpeg/gstffmpegdec.c: If ffmpeg reports 0 bytes of input data consumed, don't break out unless it also didn't pr...

Original commit message from CVS:
* ext/ffmpeg/gstffmpegdec.c:
If ffmpeg reports 0 bytes of input data consumed, don't break out
unless it also didn't produce any output. Fixes the audio in #377400
and doesn't break anything else I've tested.
Enable the mp3 parser, and set mp3 and mpeg2-video decoding autoplug
at marginal level, as they seem to both work fine now.