Fixes null pointer dereference
Fixes: signal_sigsegv_3042097_3007_cov_1741463594_non_monotone_timestamps1.mkv

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit c23a0e77)

Conflicts:

	libavcodec/h264_slice.c