Fix handling of already logged-in user in Special:Userlogin

Fix handling of already logged-in user in Special:Userlogin

Due to a forgotten return clause, the full request handling
is executed after calling successfulAction() when the user
reloads the login page while logged in. With a typical setup
case this is masked by the fact that successfulAction() does
a redirect (although it probably still impacts performance),
for less common configurations this might result in the login
form rendered below a success message, or in calling extension
hooks which should not be called.

Bug: T185702
Change-Id: If19220c4cf17b08f7e03ca0d06381067f26dda1a