gio: avoid g_object_ref(NULL) in g_dbus_object_manager_client_finalize

When establishing a D-Bus connection failed, g_dbus_object_manager_client_finalize()
calls g_object_ref(manager->priv->connection) when that pointer is NULL,
which is considered and logged as error by glib.

https://bugzilla.gnome.org/show_bug.cgi?id=732984