Fix "old style" logout

Gerrit 82634 had two bugs when used with $wgCentralAuthUseOldAutoLogin
* Reference to an uninitialized object
* Special:AutoLogin couldn't authorize the token, and so retruned an
  error instead of sending delete cookies

bug: 53962
Change-Id: I10285f7ebc8ea928f3627318ef590bfb2a692d52
Merge "Fix "old style" logout"