Fixed a problem where when a user field was requested for user 0,
a Mantis error was generated. This case occurs when displaying the history
when a defect is assigned and hence the handler name is changed from
0 to N. Hence, the history code triggers user_get_field for id 0. The code is
changed to return @null@. This needs more thought.

git-svn-id: http://mantisbt.svn.sourceforge.net/svnroot/mantisbt/trunk@1392 <a class="text" href="/?p=mantisbt.git;a=object;h=f5dc347c">f5dc347c</a>-c33d-0410-90a0-b07cc1902cb9