ffmpeg: sub2video: send a last blank frame before closing.

Fix trac ticket #3420.

ffmpeg.c

@@ -297,6 +297,8 @@ static void sub2video_flush(InputStream *ist)
 {
     int i;
 
+    if (ist->sub2video.end_pts < INT64_MAX)
+        sub2video_update(ist, NULL);
     for (i = 0; i < ist->nb_filters; i++)
         av_buffersrc_add_ref(ist->filters[i]->filter, NULL, 0);
 }