libavformat: Allow calling av_write_trailer with a NULL AVIOContext

Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/mux.c

@@ -624,7 +624,7 @@ int av_write_trailer(AVFormatContext *s)
     if (s->oformat->write_trailer)
         ret = s->oformat->write_trailer(s);
 
-    if (!(s->oformat->flags & AVFMT_NOFILE))
+    if (!(s->oformat->flags & AVFMT_NOFILE) && s->pb)
         avio_flush(s->pb);
 
 fail: