h264: check context state before decoding slice data partitions

Fixes mov_h264_aac__Demo_FlagOfOurFathers.mov.SIGSEGV.4e9.656.

Found-by: Mateusz "j00ru" Jurczyk
CC: libav-stable@libav.org
(cherry-picked from commit c1fcf563b1)

Signed-off-by: Reinhard Tartler <siretart@tauware.de>

libavcodec/h264.c

@@ -7554,6 +7554,7 @@ static int decode_nal_units(H264Context *h, const uint8_t *buf, int buf_size){
             hx->inter_gb_ptr= &hx->inter_gb;
 
             if(hx->redundant_pic_count==0 && hx->intra_gb_ptr && hx->s.data_partitioning
+               && s->current_picture_ptr
                && s->context_initialized
                && s->hurry_up < 5
                && (avctx->skip_frame < AVDISCARD_NONREF || hx->nal_ref_idc)