iwd: avoid duplicate nm_device_iwd_set_dbus_object call

Avoid calling nm_device_iwd_set_dbus_object (device, NULL) if the
dbus_object was NULL already.  Apparently gdbus guarantees that a
name-owner notification either has a NULL old owner or a NULL new owner
but can also have both old and new owner NULL.

src/devices/wifi/nm-iwd-manager.c

@@ -469,6 +469,9 @@ name_owner_changed (GObject *object, GParamSpec *pspec, gpointer user_data)
 	} else {
 		const GSList *devices, *iter;
 
+		if (!priv->running)
+			return;
+
 		priv->running = false;
 
 		devices = nm_manager_get_devices (priv->nm_manager);