Fix crash when disconnecting output

If the output being disconnected contains views, and the views are being
relocated to another output of a different size, a transaction must
occur to reconfigure them. This means by the time
container_discover_outputs is called, the output is already disabled and
wlr_output is NULL.

I considered making it check output->wlr_output, but output->enabled
should work just as well and is more descriptive.

sway/tree/container.c

@@ -985,7 +985,8 @@ void container_discover_outputs(struct sway_container *con) {
 		}
 	}
 	struct sway_output *new_output = container_get_effective_output(con);
-	double old_scale = old_output ? old_output->wlr_output->scale : -1;
+	double old_scale = old_output && old_output->enabled ?
+		old_output->wlr_output->scale : -1;
 	double new_scale = new_output ? new_output->wlr_output->scale : -1;
 	if (old_scale != new_scale) {
 		container_update_title_textures(con);