ac3dec: Don't consume more data than the actual input packet size

ac3dec: Don't consume more data than the actual input packet size

This was handled properly in the normal return case at the end
of the function, but not in this special case.

Returning a value larger than the input packet size can cause
problems for certain library users.

Returning the actual input buffer size unconditionally, since
it is not guaranteed that frame_size is set to a sensible
value at this point.

Cc: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>