wiphy: Sanity check before netdev_disassociated

The lost beacon event can be received when iwd thinks netdev is
diconnected if it was connected before iwd started, and then
netdev_disassociated will segfault.

src/wiphy.c

@@ -241,6 +241,7 @@ static void netdev_lost_beacon(struct netdev *netdev)
 		dbus_pending_reply(&netdev->connect_pending,
 				dbus_error_failed(netdev->connect_pending));
 
+	if (netdev->connected_network)
 		netdev_disassociated(netdev);
 }