rdp: be more careful to free listener on output creation failure

By the time we get to this failure we may (or may not) have implanted a
listener.

freerdp_listener_free() safely handles passed NULL pointers, so we don't
have to add any additional checks on our end.

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>

libweston/backend-rdp/rdp.c

@@ -1776,7 +1776,7 @@ rdp_backend_create(struct weston_compositor *compositor,
 
 	if (ret < 0) {
 		weston_log("Failed to register output API.\n");
-		goto err_output;
+		goto err_listener;
 	}
 
 	return b;