av_new_packet() already sets the size. And if the packet is not
allocated by av_new_packet() (which seems to be impossible atm), both
pkt->size as well as size are 0, so setting it again is unnecessary in
this scenario, too.
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>