Avoid fatal on login if username is invalid title

Avoid fatal on login if username is invalid title

Steps to reproduce: Try logging in with a 256-character username.

Expected result: Friendly error message saying the name is invalid.

Actual result: Fatal error: Call to a member function getName() on a
non-object in /var/www/git-trunk/phase3/includes/specials/SpecialUserlogin.php
on line 433

Fix: If you want to check whether something is a User object, check
whether it's a User object.  There are things other than null that don't
behave like User objects.