FIX: 0007255: APPLICATION WARNING #user_get_field() for NO_USER

New Checks -
1) check anonymous_account is set if required
2) check anonymous_account is not an ADMINISTRATOR
3) check anonymous account exists.

git-svn-id: http://mantisbt.svn.sourceforge.net/svnroot/mantisbt/trunk@4440 <a class="text" href="/?p=mantisbt.git;a=object;h=f5dc347c">f5dc347c</a>-c33d-0410-90a0-b07cc1902cb9