Always sanitize user_nicename in wp_insert_user().

Previously, a 'user_nicename' parameter passed into the function was
unsanitized. This could result in a mismatch between the sanitized nicename
generated automatically at user creation, resulting in broken author archive
permalinks.

Props joemcgill.

Fixes #29696.

git-svn-id: https://develop.svn.wordpress.org/trunk@29819 602fd350-edb4-49c9-b593-d223f7449a82