fix setAttribute ATTR_TIMEOUT when $this->_client is not initialized

Hi,

Initializing crate PDO with timeout in the $options array throws and error:

```php
$this->_pdo = new PDO('crate:' . $descriptor['host'] . ':' . $descriptor['port'], null, null, [
            PDO::ATTR_TIMEOUT => 100
        ]);
```

```php
Fatal error: Call to a member function setTimeout() on a non-object in /opt/web/gitlab/php-jabol-logs/vendor/crate/crate-pdo/src/Crate/PDO/PDO.php on line 256 
```

because $this->_client is not initilized yet.