Fix undefined variable in Article Model

Fixing fatal error during article saving as reported by Brian in #5159

````
Notice: Undefined variable: app in administrator/components/com_content/models/article.php on line 522

Fatal error: Call to a member function enqueueMessage() on a non-object in administrator/components/com_content/models/article.php on line 522
````

To reproduce you likely need to save an article without specifying an alias. If the generated alias already exists, Joomla tries to issue a warning, which triggers that error.
At least that's what I would expect from looking at the code.