network/netdev: skip processing netdev if it is already detached

No functional change, as currently networkd detaches NetDev objects only
on stop (or invalid .netdev file is loaded).
Preparation for later commits.

src/network/networkd-queue.c

@@ -193,6 +193,7 @@ int netdev_queue_request(
         int r;
 
         assert(netdev);
+        assert(netdev->manager);
 
         r = request_new(netdev->manager, NULL, REQUEST_TYPE_NETDEV_INDEPENDENT,
                         netdev, (mfree_func_t) netdev_unref,