call db.close instead of this.close to avoid recursion (#6056)

std/php7/_std/sys/db/Mysql.hx

@@ -69,7 +69,7 @@ private class MysqlConnection implements Connection {
 	}
 
 	public function close() : Void {
-		close();
+		db.close();
 	}
 
 	public function escape( s : String ) : String {