GtkPlacesSidebar: do not unref mount

Do not unref mount has we grabbed its pointer without taking
a reference.
Otherwise the following call to g_list_free_full on network_mounts
frees it a second time. Segfault in nautilus ensue.

https://bugzilla.gnome.org/show_bug.cgi?id=700979

gtk/gtkplacessidebar.c

@@ -1123,7 +1123,6 @@ update_places (GtkPlacesSidebar *sidebar)
 			   name, icon, mount_uri,
 			   NULL, NULL, mount, 0, tooltip);
 		g_object_unref (root);
-		g_object_unref (mount);
 		g_object_unref (icon);
 		g_free (name);
 		g_free (mount_uri);