avcodec/h264_slice: Disable slice threads if there are multiple access units in a packet

Fixes null pointer dereference
Fixes part of Ticket4977
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 9368d2da3d33cac845f2fdf663df500b53625c5e)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/h264_slice.c

@@ -1177,6 +1177,15 @@ int ff_h264_decode_slice_header(H264Context *h, H264SliceContext *sl)
                 av_log(h->avctx, AV_LOG_ERROR, "Too many fields\n");
                 return AVERROR_INVALIDDATA;
             }
+            if (h->max_contexts > 1) {
+                if (!h->single_decode_warning) {
+                    av_log(h->avctx, AV_LOG_WARNING, "Cannot decode multiple access units as slice threads\n");
+                    h->single_decode_warning = 1;
+                }
+                h->max_contexts = 1;
+                return SLICE_SINGLETHREAD;
+            }
+
             if (h->cur_pic_ptr && FIELD_PICTURE(h) && h->first_field) {
                 ret = ff_h264_field_end(h, h->slice_ctx, 1);
                 h->current_slice = 0;