Add a nonce check on the confirm links in the Network Admin stage one.

git-svn-id: http://svn.automattic.com/wordpress/trunk@16547 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/network/edit.php

@@ -311,6 +311,7 @@ switch ( $_GET['action'] ) {
 
 	// Common
 	case 'confirm':
+		check_admin_referer( 'confirm' );
 		if ( !headers_sent() ) {
 			nocache_headers();
 			header( 'Content-Type: text/html; charset=utf-8' );