Return WP_Error from wp_update_user() on a non-existent user, avoiding a fatal error in the process.

Merges [23210] to the 3.5 branch.

props n7studios, SergeyBiryukov.
fixes #22858.



git-svn-id: http://core.svn.wordpress.org/branches/3.5@23211 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/user.php

@@ -1452,6 +1452,8 @@ function wp_update_user($userdata) {
 
 	// First, get all of the original fields
 	$user_obj = get_userdata( $ID );
+	if ( ! $user_obj )
+		return new WP_Error( 'invalid_user_id', __( 'Invalid user ID.' ) );
 
 	$user = $user_obj->to_array();