This unbreaks muxing-encoding
Example:
ffmpeg -i matrixbench_mpeg2.mpg new.avi

-rw-r----- 1 michael michael 226035354 Jan  1 16:27 new.avi
-rw-r----- 1 michael michael  10016802 Jan  1 16:28 ref.avi

Also av_get_audio_frame_duration() itself uses frame_size

This reverts commit 29e6606e, reversing
changes made to 53448461.