avcodec/hnm4video: change width/height to int

Fixes hypothetical integer overflows
Related to CID1135770 & CID1135771

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/h264: fix code that blindly dereferences NULL DPB

Fixes mixed flushing and decoding NULL packets
Found-by: wm4

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