sessions: fix error when an anonymous user tried to logout.

Fix an exception when a user who was already logged out tried to logout
again.

app/logical/session_loader.rb

@@ -31,6 +31,7 @@ class SessionLoader
 
   def logout
     session.delete(:user_id)
+    return if CurrentUser.user.is_anonymous?
     UserEvent.create_from_request!(CurrentUser.user, :logout, request)
   end