avcodec/mp3: fix skipping zeros

avcodec/mp3: fix skipping zeros

Commits 43bc5cf9 and c5371f77 add code for skipping initial zeros in mp3
packets. This code forgot to report to the user that data was skipped at
all.

Since audio codecs allow partial packet decoding, the user application
has to rely on the return value. It will remove the data reported as
consumed by the decoder, and feed it to the decoder again. This resulted
in the mp3 frame after the zero region to be decoded over and over
again, until the zero region was finally skipped by the application.

Fix this by including the amount of skipped bytes to the number of
consumed bytes returned by the decode call.

Fixes trac ticket #4890.