Index: wp-includes/user.php
===================================================================
--- wp-includes/user.php	(revision: 21966)
+++ wp-includes/user.php	(working copy)
@@ -117,9 +117,7 @@
  * Authenticate the user using the WordPress auth cookie.
  */
 function wp_authenticate_cookie($user, $username, $password) {
-	if ( is_a($user, 'WP_User') ) { return $user; }
-
-	if ( empty($username) && empty($password) ) {
+	if ( !is_a($user, 'WP_User') && empty($username) && empty($password) ) {
 		$user_id = wp_validate_auth_cookie();
 		if ( $user_id )
 			return new WP_User($user_id);