lavf: dealloc private options in av_write_trailer

Fix the iformat/oformat typo.

libavformat/utils.c

@@ -3261,7 +3261,7 @@ fail:
         av_freep(&s->streams[i]->priv_data);
         av_freep(&s->streams[i]->index_entries);
     }
-    if (s->iformat && s->iformat->priv_class)
+    if (s->oformat->priv_class)
         av_opt_free(s->priv_data);
     av_freep(&s->priv_data);
     return ret;