When updating a user, invalidate its 'userslugs' cache.

`user_nicename` can be changed via `wp_update_user()`, so we invalidate just
to be safe.

Props thebrandonallen.
Fixes #35750.

git-svn-id: https://develop.svn.wordpress.org/trunk@36482 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/user.php

@@ -1682,6 +1682,7 @@ function wp_update_user($userdata) {
 	}
 
 	wp_cache_delete( $user['user_email'], 'useremail' );
+	wp_cache_delete( $user['user_nicename'], 'userslugs' );
 
 	// Merge old and new fields with new fields overwriting old ones.
 	$userdata = array_merge( $user, $userdata );