Added NULL check for engine->port.

This is required to handle REMOVE_PID messages if router engine
initialization is incomplete.

src/nxt_router.c

@@ -944,9 +944,11 @@ nxt_router_remove_pid_handler(nxt_task_t *task, nxt_port_recv_msg_t *msg)
 
     nxt_queue_each(engine, &nxt_router->engines, nxt_event_engine_t, link0)
     {
+        if (nxt_fast_path(engine->port != NULL)) {
             nxt_port_post(task, engine->port, nxt_router_app_process_remove_pid,
                           msg->u.data);
         }
+    }
     nxt_queue_loop;
 
     if (msg->port_msg.stream == 0) {