Fixes possible null pointer exception. Thanks to Paul for the fix !

git-svn-id: http://csipsimple.googlecode.com/svn/trunk/CSipSimple@2099 9f815046-5998-e9c0-b7e2-ac03a23edfa4
Fixes issue 2190. Thanks to  bakfitty for the very helpful investigation and fix.

git-svn-id: http://csipsimple.googlecode.com/svn/trunk/CSipSimple@2102 9f815046-5998-e9c0-b7e2-ac03a23edfa4