oops: Fix fatal error in SLA page

Fixes #1660

include/staff/slaplans.inc.php

@@ -34,6 +34,7 @@ $count = SLA::objects()->count();
 $qstr = '&'. Http::build_query($qs);
 $qs += array('sort' => $_REQUEST['sort'], 'order' => $_REQUEST['order']);
 
+$pageNav = new Pagenate($count, $page, PAGE_LIMIT);
 $pageNav->setURL('slas.php', $qs);
 $showing = $pageNav->showing().' '._N('SLA plan', 'SLA plans', $count);
 $qstr .= '&order='.($order=='DESC' ? 'ASC' : 'DESC');