Use get_current_site() instead of the $current_site global in wp-admin/admin-header.php and is_main_network().

fixes #25158.
Built from https://develop.svn.wordpress.org/trunk@26235


git-svn-id: http://core.svn.wordpress.org/trunk@26142 1a063a9b-81f0-0310-95a4-ce76da25c4cd