oggdemux: fix chain leak

Don't leak the building_chain when destroying.

Fix leaks with the validate.http.playback.reverse_playback.vorbis_theora_1_ogg
scenario.

https://bugzilla.gnome.org/show_bug.cgi?id=748964

ext/ogg/gstoggdemux.c

@@ -2272,6 +2272,9 @@ gst_ogg_demux_finalize (GObject * object)
 
   gst_flow_combiner_free (ogg->flowcombiner);
 
+  if (ogg->building_chain)
+    gst_ogg_chain_free (ogg->building_chain);
+
   G_OBJECT_CLASS (parent_class)->finalize (object);
 }