Fixed some php warnings in update script that appear when a version without...

Fixed some php warnings in update script that appear when a version without multiserver replication is updated.

install/update.php

@@ -89,10 +89,10 @@ $conf['mysql']['charset'] = 'utf8';
 $conf["mysql"]["ispconfig_user"] = $conf_old["db_user"];
 $conf["mysql"]["ispconfig_password"] = $conf_old["db_password"];
 
-$conf["mysql"]["master_host"] = $conf_old["dbmaster_host"];
-$conf["mysql"]["master_database"] = $conf_old["dbmaster_database"];
-$conf["mysql"]["master_ispconfig_user"] = $conf_old["dbmaster_user"];
-$conf["mysql"]["master_ispconfig_password"] = $conf_old["dbmaster_password"];
+if(isset($conf_old["dbmaster_host"])) $conf["mysql"]["master_host"] = $conf_old["dbmaster_host"];
+if(isset($conf_old["dbmaster_database"])) $conf["mysql"]["master_database"] = $conf_old["dbmaster_database"];
+if(isset($conf_old["dbmaster_user"])) $conf["mysql"]["master_ispconfig_user"] = $conf_old["dbmaster_user"];
+if(isset($conf_old["dbmaster_password"])) $conf["mysql"]["master_ispconfig_password"] = $conf_old["dbmaster_password"];
 
 // Resolve the IP address of the mysql hostname.
 if(!$conf['mysql']['ip'] = gethostbyname($conf['mysql']['host'])) die('Unable to resolve hostname'.$conf['mysql']['host']);