Check if data is defined

Otherwise when adding a user as subadmin this would throw the following JS error:

```
users.js?v=bc2aae7…:500 Uncaught TypeError: Cannot read property 'message' of undefined(…)(anonymous function) @ users.js?v=bc2aae7…:500j @ jquery.js:3099fireWith @ jquery.js:3211x @ jquery.js:8264(anonymous function) @ jquery.js:8605
```

Signed-off-by: Lukas Reschke <lukas@statuscode.ch>

settings/js/users/users.js

@@ -497,7 +497,7 @@ var UserList = {
 					group: group
 				},
 				function (response) {
-					if (response.data.message) {
+					if (response.data !== undefined && response.data.message) {
 						OC.Notification.show(response.data.message);
 					}
 				}