avcodec/h264: Check *log2_weight_denom

Fixes undefined behavior
Fixes: signal_sigsegv_14768d2_2248_cov_3629497219_h264_h264___pi_20070614T182942.h264
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 61296d41e2de3b41304339e4631dd44c2e15f805)

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
1 file changed