Fix PHP notices "Undefined index" in as_login.php. (#392)
Fix PHP notices "Undefined index" in as_login.php. (#392)

Sometimes appear in error logs as:

PHP Notice:  Undefined index: ReturnTo in .../modules/core/www/as_login.php on line 9
PHP Notice:  Undefined index: AuthId in .../modules/core/www/as_login.php on line 13
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