Login and Registration: Only call `get_blog_details()` in `wp_lostpassword_url()` on Multisite.

The function does not exist on single site.

Follow-up to [48672].

See #39311.
Built from https://develop.svn.wordpress.org/trunk@48673


git-svn-id: https://core.svn.wordpress.org/trunk@48435 1a063a9b-81f0-0310-95a4-ce76da25c4cd