avcodec/gdv: Remove dead check

avcodec/gdv: Remove dead check

At the end of its decode function, the decoder sets *got_frame to 1 and
then checks whether ret is < 0; if so, it is returned, otherwise
avpkt->size is. But it is impossible for ret to be < 0 here and if it
were, it would be nonsense to set *got_frame to 1 before this. Therefore
just return avpkt->size unconditionally.

Fixes Coverity issue #1439730.

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>