don't crash when initializing more monitors than supported

Fixes https://codeberg.org/dwl/dwl/issues/726

dwl.c

@@ -1262,6 +1262,10 @@ createmon(struct wl_listener *listener, void *data)
 	 * output (such as DPI, scale factor, manufacturer, etc).
 	 */
 	m->scene_output = wlr_scene_output_create(scene, wlr_output);
+
+	if (!wlr_output->enabled)
+		return;
+
 	if (m->m.x == -1 && m->m.y == -1)
 		wlr_output_layout_add_auto(output_layout, wlr_output);
 	else