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)

Conflicts:

	libavcodec/h264_slice.c

1 files changed, 9 insertions, 0 deletions

diff --git a/libavcodec/h264_slice.c b/libavcodec/h264_slice.c
index 968e3ecb83..10f4d77380 100644
--- a/libavcodec/h264_slice.c
+++ b/libavcodec/h264_slice.c
@@ -1159,6 +1159,15 @@ int ff_h264_decode_slice_header(H264Context *h, H264SliceContext *sl)
 
     if (first_mb_in_slice == 0) { // FIXME better field boundary detection
         if (h->current_slice) {
+            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) {
                 ff_h264_field_end(h, h->slice_ctx, 1);
                 h->current_slice = 0;