Coding Standards: Replace redundant check in wpmu_validate_blog_signup().

The `is_object()` check would always be true in the second part of the conditional. This commit adds an `instanceof WP_User` check instead before accessing the `user_login` property.

Follow-up to [41963], [https://mu.trac.wordpress.org/changeset/550 mu:550], [https://mu.trac.wordpress.org/changeset/1958 mu:1958], [12603].

Props justlevine.
See #63268.

git-svn-id: https://develop.svn.wordpress.org/trunk@60450 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

src/wp-includes/ms-functions.php

@@ -730,7 +730,7 @@ function wpmu_validate_blog_signup( $blogname, $blog_title, $user = '' ) {
 	 * unless it's the user's own username.
 	 */
 	if ( username_exists( $blogname ) ) {
-		if ( ! is_object( $user ) || ( is_object( $user ) && $user->user_login !== $blogname ) ) {
+		if ( ! $user instanceof WP_User || $user->user_login !== $blogname ) {
 			$errors->add( 'blogname', __( 'Sorry, that site is reserved!' ) );
 		}
 	}