Don't call $this->config directly, instead call $this->getZeroConfig()

Fatal error: Call to a member function showImages() on a non-object at
/srv/mediawiki/php-1.25wmf6/extensions/ZeroBanner/includes/PageRendering.php
on line 345

Change-Id: If94bf403d869f910e338eca4d23404ff1177264b
Merge "Don't call $this->config directly, instead call $this->getZeroConfig()"