Fixes crash

Found-by: iive
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 8710ee11)

Conflicts:

	libavcodec/h264_slice.c
(cherry picked from commit cb44d69665596d2f19e032fc852162b0a6de0562)
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>