Login and Registration: Correct the logic for determining the path to...

Login and Registration: Correct the logic for determining the path to `wp-login.php` in `wp_lostpassword_url()`.

Follow-up to [48672], [48673].

See #39311.
Built from https://develop.svn.wordpress.org/trunk@48674


git-svn-id: https://core.svn.wordpress.org/trunk@48436 1a063a9b-81f0-0310-95a4-ce76da25c4cd