Use get_current_site() instead of the $current_site global when possible.

props jeremyfelt.
fixes #25158.

git-svn-id: https://develop.svn.wordpress.org/trunk@26120 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/ms-default-constants.php

@@ -45,7 +45,7 @@ function ms_upload_constants() {
  * @since 3.0.0
  */
 function ms_cookie_constants(  ) {
-	global $current_site;
+	$current_site = get_current_site();
 
 	/**
 	 * @since 1.2.0