Otherwise the h263p decoder can try to copy overread bytes, even though
buffer is NULL.
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
(cherry picked from commit 6a69a175)
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>