fix objc utility bug - pointer assign to null instead of it value

Bug fix in Objective-C parser.
Assign null to the pointer value instead of the pointer itself.

Bug report on jira <a href="http://jira.codehaus.org/browse/ENUNCIATE-805">ENUNCIATE-805</a>

Martin Magakian,
<a href="http://doduck.com">doduck prototype</a>