avcodec/aac_ac3_parser: account for data already in the parsing buffer

avcodec/aac_ac3_parser: account for data already in the parsing buffer

If a frame starts very close to a packet boundary, the start code may
already have been added to the parsing buffer, indicated by a small
negative value of "i", while the header is still being tracked in the
"state" variable.

Reduce the remaining size accordingly, otherwise trying to find the next
frame could skip over the frame header and lump two frames together as
one.