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

libavcodec/h264.c

@@ -3848,6 +3848,7 @@ again:
                 if (hx->redundant_pic_count == 0 &&
                     hx->intra_gb_ptr &&
                     hx->s.data_partitioning &&
+                    s->current_picture_ptr &&
                     s->context_initialized &&
                     (avctx->skip_frame < AVDISCARD_NONREF || hx->nal_ref_idc) &&
                     (avctx->skip_frame < AVDISCARD_BIDIR  ||