Segfault on session php7.3

PHP Version 7.3.0-2+ubuntu16.04.1+deb.sury.org+1
MessagePack Support enabled
Session Support enabled
extension Version 2.0.4-dev
header Version 0.5.4

```
<?php

session_start();
$_SESSION['a'] = 1;
session_write_close();

ini_set('session.serialize_handler', 'msgpack');
session_start();
```
result: segfault

