Fix fatal error during ?action=slide

"Fatal error: Call to a member function getHeadItems() on a non-object in .../extensions/S5SlideShow/S5SlideShow.class.php on line 371"

This happened when $wgParser didn't unstub itself before running S5 action hook;
So the error reproduced on installations where no extension used $wgParser before first call init...
For example, on a clear one :-)