h264: disallow single unpaired fields as references of frames

Fixes deadlock

libavcodec/h264_refs.c

@@ -293,12 +293,14 @@ int ff_h264_decode_ref_pic_list_reordering(H264Context *h){
     }
     for(list=0; list<h->list_count; list++){
         for(index= 0; index < h->ref_count[list]; index++){
-            if (!h->ref_list[list][index].f.data[0]) {
+            if (   !h->ref_list[list][index].f.data[0]
+                || (!FIELD_PICTURE && (h->ref_list[list][index].reference&3) != 3)) {
                 int i;
                 av_log(h->avctx, AV_LOG_ERROR, "Missing reference picture, default is %d\n", h->default_ref_list[list][0].poc);
                 for (i=0; i<FF_ARRAY_ELEMS(h->last_pocs); i++)
                     h->last_pocs[i] = INT_MIN;
-                if (h->default_ref_list[list][0].f.data[0])
+                if (h->default_ref_list[list][0].f.data[0]
+                    && !(!FIELD_PICTURE && (h->default_ref_list[list][index].reference&3) != 3))
                     COPY_PICTURE(&h->ref_list[list][index], &h->default_ref_list[list][0]);
                 else
                     return -1;