[ticket/13234] Never allow autologin/remember me to modify the userid

Ascraeus version of 64d97d0787a63b3c646f89237574ac566ed89c50 commit

PHPBB3-13234
[ticket/13234] Correctly verify that user_id is set in user data array

PHPBB3-13234
[ticket/13234] Merge branch 'ticket/13234' into ticket/13234-2

* ticket/13234:
  [ticket/13234] Fix conditions and CS

Conflicts:
	phpBB/phpbb/session.php

PHPBB3-13234