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>