replace UserChecker checkPostAuth by checkPreAuth in LoginManager

Fixes https://github.com/FriendsOfSymfony/FOSUserBundle/issues/1413

The checks against login in are in [userChecker checkPreAuth]( https://github.com/symfony/symfony/blob/master/src/Symfony/Component/Security/Core/User/UserChecker.php ), not checkPostAuth , since the content of those 2 checks was fixed in https://github.com/symfony/symfony/pull/9902.

NB: Where is a good place for checkPostAuth ? Should it be handled within this bundle ?