avformat/rtpdec_h264: fix null pointer dereferences

Fixes CID733716

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avformat/rtpenc: check av_packet_get_side_data() return, fix null ptr dereference

Fixes CID1035715

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