lavf: flush the output AVIOContext in av_write_trailer().

This is consistent with stdio and is what we want to do in all cases.

Fixes a bug in the voc muxer which didn't flush in write_trailer()
previously. This is the cause of the change in the test results.

libavformat/utils.c

@@ -3270,6 +3270,10 @@ int av_write_trailer(AVFormatContext *s)
 
     if (s->oformat->write_trailer)
         ret = s->oformat->write_trailer(s);
+
+    if (!(s->oformat->flags & AVFMT_NOFILE))
+        avio_flush(s->pb);
+
 fail:
     for (i = 0; i < s->nb_streams; i++) {
         av_freep(&s->streams[i]->priv_data);