Added destructor to mysql lib

http://bugtracker.ispconfig.org/index.php?do=details&task_id=1671

interface/lib/classes/db_mysql.inc.php

@@ -54,6 +54,10 @@ class db {
 		//$this->connect();
 	}
 
+	public function __destruct() {
+		$this->closeConn();
+	}
+
 	/**  Error handler */
 	public function updateError($location)
     {