h264: error out on unset current_picture_ptr for h->current_slice > 0

Fixes a segfault with fuzzed sample sample_varPAR_s11622_r001-02.avi.

CC: libav-stable@libav.org

libavcodec/h264.c

@@ -2624,6 +2624,11 @@ static int decode_slice_header(H264Context *h, H264Context *h0)
             s->picture_structure = last_pic_structure;
             s->dropable          = last_pic_dropable;
             return AVERROR_INVALIDDATA;
+        } else if (!s->current_picture_ptr) {
+            av_log(s->avctx, AV_LOG_ERROR,
+                   "unset current_picture_ptr on %d. slice\n",
+                   h0->current_slice + 1);
+            return AVERROR_INVALIDDATA;
         }
     } else {
         /* Shorten frame num gaps so we don't have to allocate reference