avformat/mp3dec: fix gapless audio support

The code already had skipping of initial padding, but discarding
trailing frame padding was missing.

This is somewhat questionable, because it will make the decoder discard
any data after the declared file size in the LAME header. But note that
skipping full frames at the end of the stream is required. Encoders
actually create such files.

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