xwayland/xwm: Allocate to the right variable

837060f894 made a change to only allocate the xcb_window_t array if it
was not going to be zero-sized. It accidentally shadowed the variable so
the output of the allocation would never be used.

Regressed-by: https://gitlab.freedesktop.org/wlroots/wlroots/-/merge_requests/4563

xwayland/xwm.c

@@ -292,7 +292,7 @@ static void xwm_set_net_client_list(struct wlr_xwm *xwm) {
 
 	xcb_window_t *windows = NULL;
 	if (mapped_surfaces > 0) {
-		xcb_window_t *windows = malloc(sizeof(*windows) * mapped_surfaces);
+		windows = malloc(sizeof(*windows) * mapped_surfaces);
 		if (!windows) {
 			return;
 		}