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>