ffs: revert to the old ffmpeg api in ffs_adec()
The new function avcodec_decode_audio4() relies on the new
AFPacket.nb_samples. So I cannot use frame->linesize[0] to
get the size of the audio data. This means I should just
redo most of the things done in the avcodec_decode_audio3()
wrapper around avcodec_decode_audio4(); it is undeniably
ugly. So I'll stick with avcodec_decode_audio3() for now.
I hope one day the ffmpeg people learn how to design the API.
Reported-by: Sara Fauzia <sfauzia@fas.harvard.edu>