vtdec: flush decoder on stop

fix a memory leak if the decoder shut down before EOS

https://bugzilla.gnome.org/show_bug.cgi?id=787311

sys/applemedia/vtdec.c

@@ -187,6 +187,8 @@ gst_vtdec_stop (GstVideoDecoder * decoder)
 {
   GstVtdec *vtdec = GST_VTDEC (decoder);
 
+  gst_vtdec_push_frames_if_needed (vtdec, TRUE, TRUE); 
+
   if (vtdec->input_state)
     gst_video_codec_state_unref (vtdec->input_state);
   vtdec->input_state = NULL;