MDL-58220 oauth2: Fix account linking

The username passed to the confirm account link request is the oauth username,
which may be different to the moodle username so can't be used to look up the user record.

auth/oauth2/confirm-linkedlogin.php

@@ -40,8 +40,8 @@ if ($confirmed) {
 
     // The user has confirmed successfully, let's log them in.
 
-    if (!$user = get_complete_user_data('username', $username)) {
-        print_error('cannotfinduser', '', '', s($username));
+    if (!$user = get_complete_user_data('id', $userid)) {
+        print_error('cannotfinduser', '', '', $userid);
     }
 
     if (!$user->suspended) {