gdbus: Close private connection if setup fails

Private connection should be properly closed with dbus_connection_close
otherwise libdbus exits with the following error:

  'The last reference on a connection was dropped without closing the
   connection. This is a bug in an application. See
   dbus_connection_unref() documentation for details. Most likely, the
   application was supposed to call dbus_connection_close(), since this
   is a private connection.'
diff --git a/gdbus/mainloop.c b/gdbus/mainloop.c
index 3e88eac..b90a844 100644
--- a/gdbus/mainloop.c
+++ b/gdbus/mainloop.c
@@ -322,6 +322,7 @@
 		return NULL;
 	if (setup_bus(conn, name, error) == FALSE) {
+		dbus_connection_close(conn);
 		return NULL;