Utilities::generateUserIdentifier: Require UserID to be set in $state.

If UserID isn't set in $state, it means that we were unable to retrieve
it previously. Trying again isn't likely to succeed.

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@2377 44740490-163a-0410-bde0-09ae8108e29a