xwayland: Clean up if Xwayland fails to start

When running wlroots compositors with Xwayland executable bits
unset, if DISPLAY is set to the display number wlroots has set
up, then X and gtk clients (at least) hang when they are ran.
X clients should fail with an error and exit while gtk clients
should fall back to wayland backend and run correctly. This is
because wlroots opened sockets for Xwayland but wasn't closing
them if Xwayland failed to start.

xwayland/xwayland.c

@@ -219,18 +219,17 @@ static int xserver_handle_ready(int signal_number, void *data) {
 			continue;
 		}
 		wlr_log_errno(WLR_ERROR, "waitpid for Xwayland fork failed");
-		return 1;
+		goto error;
 	}
 	if (stat_val) {
 		wlr_log(WLR_ERROR, "Xwayland startup failed, not setting up xwm");
-		return 1;
+		goto error;
 	}
 	wlr_log(WLR_DEBUG, "Xserver is ready");
 
 	wlr_xwayland->xwm = xwm_create(wlr_xwayland);
 	if (!wlr_xwayland->xwm) {
-		xwayland_finish_server(wlr_xwayland);
-		return 1;
+		goto error;
 	}
 
 	if (wlr_xwayland->seat) {
@@ -254,6 +253,13 @@ static int xserver_handle_ready(int signal_number, void *data) {
 	wl_signal_init(&wlr_xwayland->events.ready);
 
 	return 1; /* wayland event loop dispatcher's count */
+error:
+	/* clean up */
+	wlr_xwayland_set_seat(wlr_xwayland, NULL);
+	xwayland_finish_server(wlr_xwayland);
+	xwayland_finish_display(wlr_xwayland);
+
+	return 1;
 }
 
 static int xwayland_socket_connected(int fd, uint32_t mask, void* data){