h264_refs: check for multiple mismatching long term ref assignments

Fixes failure of the assert added a few hours ago.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/h264_refs.c

@@ -626,6 +626,14 @@ int ff_h264_execute_ref_pic_marking(H264Context *h, MMCO *mmco, int mmco_count)
                      */
 
             if (h->long_ref[mmco[i].long_arg] != h->cur_pic_ptr) {
+                if (h->cur_pic_ptr->long_ref) {
+                    for(j=0; j<16; j++) {
+                        if(h->long_ref[j] == h->cur_pic_ptr) {
+                            remove_long(h, j, 0);
+                            av_log(h->avctx, AV_LOG_ERROR, "mmco: cannot assign current picture to 2 long term references\n");
+                        }
+                    }
+                }
                 av_assert0(!h->cur_pic_ptr->long_ref);
                 remove_long(h, mmco[i].long_arg, 0);
                 if (remove_short(h, h->cur_pic_ptr->frame_num, 0)) {