avcodec/h264_slice: More complete cleanup in h264_slice_header_init()

Fixes null pointer dereference
Fixes Ticket3873

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 1fa35e4352cc39894987e14de464e3d72b55739f)

Conflicts:

	libavcodec/h264_slice.c
1 file changed