network: refuse further requests when manager is in MANAGER_STOPPED

In that case, requests will never be processed anyway. But further more,
we cannot call link_ref() at that stage. Otherwise, we trigger assertion.

src/network/networkd-queue.c

@@ -142,6 +142,15 @@ static int request_new(
         assert(manager);
         assert(process);
 
+        /* Note, requests will be processed only when the manager is in MANAGER_RUNNING. If a new operation
+         * is requested when the manager is in MANAGER_TERMINATING or MANAGER_RESTARTING, the request will be
+         * successfully queued but will never be processed. Then, here why we refuse new requests when the
+         * manager is in MANAGER_STOPPED? This is because we cannot call link_ref() in that case, as this may
+         * be called during link_free(), that means the reference counter of the link is already 0 and
+         * calling link_ref() below triggers assertion. */
+        if (manager->state == MANAGER_STOPPED)
+                return -EBUSY;
+
         req = new(Request, 1);
         if (!req)
                 return -ENOMEM;
@@ -426,6 +435,12 @@ int remove_request_add(
         assert(netlink);
         assert(message);
 
+        /* Unlike request_new(), remove requests will be also processed when the manager is in
+         * MANAGER_TERMINATING or MANAGER_RESTARTING. When the manager is in MANAGER_STOPPED, we cannot
+         * queue new remove requests anymore with the same reason explained in request_new(). */
+        if (manager->state == MANAGER_STOPPED)
+                return 0;
+
         req = new(RemoveRequest, 1);
         if (!req)
                 return -ENOMEM;