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>
(cherry picked from commit c2e8691c07)

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

ffplay.c

@@ -1921,6 +1921,7 @@ 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);