gdbus: Don't refresh objects/props if disconnected

gdbus: Don't refresh objects/props if disconnected

If g_dbus_client_set_proxy_handlers gets called from within a
proxy_removed callback, the code may end up refreshing the proxy's
properties and incorrectly access the client's proxy_list as it gets
freed. This patch fixes this, so that get_managed_objects does nothing
if it gets called during a service disconnect.