[1.2.X] Fixed #14242 - UserChangeForm subclasses without 'user_permissions' field causes KeyError

  
This was a regression introduced by [13684]
  
Thanks to adammckerlie@gmail.com for report.

Backport of [13702] from trunk.


git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@13704 bcc190cf-cafb-0310-a4f2-bffc1f526a37