Update SecurityUtil.php

Ensure that uid is always an integer: 


Q	A
Bug fix?	no
New feature?	yes
BC breaks?	no
Deprecations?	no
Tests pass?	yes
Fixed tickets	
License	MIT
Doc PR	no
Dont send an welcome email to new users function added, closes #731.
Merge pull request #863 from phaidon/new-user-notification-add

Dont send an welcome email to new users function added, closes #731.
Fixed wrong classnames, closes #864.
Merge pull request #865 from cmfcmf/user-delete-fix

Fixed wrong classnames, closes #864.
Handle checkboxes by z-form-error, closes #636
Merge pull request #856 from rgasch/patch-12

Update SecurityUtil.php
Merge pull request #869 from cmfcmf/checkbox-fix

Handle checkboxes by z-form-error, closes #636