Resets current user.

git-svn-id: http://svn.redmine.org/redmine@13447 e93f8b46-1217-0410-a6f0-8f06a7374b81
Fixed that Issue.visible with an unsaved user triggers an SQL error.

git-svn-id: http://svn.redmine.org/redmine@13448 e93f8b46-1217-0410-a6f0-8f06a7374b81