lavf/mov: Audio and fourcc 0x00000000

lavf/mov: Audio and fourcc 0x00000000

In many older QuickTime files, the audio format, or "fourcc", is
0x00000000. The QuickTime File Format Specification states the following
regarding this situation:

"This format descriptor should not be used, but may be found in some
files. Samples are assumed to be stored in either 'raw ' or 'twos'
format, depending on the sample size field in the sound description."

MPlayer handles this logic by itself, but FFmpeg/FFplay currently does
not.

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>