mpegaudiodec/mp3on4: fix buffer size.

The larger (and really ugly) size is not needed anymore.

Found-by: Justin Ruggles
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
ac3dec: split out pointer update loop for saftey

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>