avfiltergraph: free the sink_links heap.

libavfilter/avfiltergraph.c

@@ -55,6 +55,7 @@ void avfilter_graph_free(AVFilterGraph **graph)
         return;
     for (; (*graph)->filter_count > 0; (*graph)->filter_count--)
         avfilter_free((*graph)->filters[(*graph)->filter_count - 1]);
+    av_freep(&(*graph)->sink_links);
     av_freep(&(*graph)->scale_sws_opts);
     av_freep(&(*graph)->filters);
     av_freep(graph);