Make avfilter_graph_free() free the graph.

Make avfilter_graph_free() free not only the internal structures, but
also the allocated graph, and set the graph pointer to NULL for
increased safety.

Simplify usage.

Signed-off-by: Mans Rullgard <mans@mansr.com>
(cherry picked from commit 4359288c565705d1734f63d277f8918ee5af5e54)

ffplay.c

@@ -1899,8 +1899,7 @@ static int video_thread(void *arg)
     }
  the_end:
 #if CONFIG_AVFILTER
-    avfilter_graph_free(graph);
-    av_freep(&graph);
+    avfilter_graph_free(&graph);
 #endif
     av_free(frame);
     return 0;