ffplay: flush codec buffers before freeing filters

We do this to ensure that input_get_buffer is not called from a
frame_worker_thread of a multithreaded decoder when we already freed the
filters.

Fixes occasional segfaults on video stream change.
Signed-off-by: Marton Balint <cus@passwd.hu>

ffplay.c

@@ -1915,6 +1915,7 @@ static int video_thread(void *arg)
             stream_toggle_pause(is);
     }
  the_end:
+    avcodec_flush_buffers(is->video_st->codec);
 #if CONFIG_AVFILTER
     av_freep(&vfilters);
     avfilter_graph_free(&graph);