st-widget: Chain up in the dispose handler for accessibility objects

https://bugzilla.gnome.org/show_bug.cgi?id=678737

src/st/st-widget.c

@@ -2502,6 +2502,8 @@ st_widget_accessible_dispose (GObject *gobject)
       g_object_unref (self->priv->current_label);
       self->priv->current_label = NULL;
     }
+
+  G_OBJECT_CLASS (st_widget_accessible_parent_class)->dispose (gobject);
 }
 
 static void