Fixed #12325 -- Allows zero values for moderate_after and close_after fields...

Fixed #12325 -- Allows zero values for moderate_after and close_after fields of comment moderators. Thanks, Gabriel Hurley.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@14556 bcc190cf-cafb-0310-a4f2-bffc1f526a37