Extra isset test

server/plugins-available/apache2_plugin.inc.php

@@ -1831,7 +1831,7 @@ class apache2_plugin {
 		//* Add vhost for IPv6 IP
 		if($data['new']['ipv6_address'] != '') {
 			//* rewrite ipv6 on mirrors
-			if ($conf['serverconfig']['web']['vhost_rewrite_v6'] == 'y') {
+			if (isset($conf['serverconfig']['web']['vhost_rewrite_v6']) && $conf['serverconfig']['web']['vhost_rewrite_v6'] == 'y') {
 				if (isset($conf['serverconfig']['server']['v6_prefix']) && $conf['serverconfig']['server']['v6_prefix'] <> '') {
 					$explode_v6prefix=explode(':', $conf['serverconfig']['server']['v6_prefix']);
 					$explode_v6=explode(':', $data['new']['ipv6_address']);