StandardClient()->protected function getTag(DOMNode $node, $tag) triggers notice when action is cancelled

When an iDeal transaction is cancelled, the XML return is different and does not contain all the parsed tags. This causes a PHP Notice in the getTag parser

```
[Mon Aug 26 22:42:49 2013] [error] [client <>] PHP Notice:  Trying to get property of non-object in /var/www/ewk/vendor/slm/ideal-payment/src/SlmIdealPayment/Client/StandardClient.php on line 307
```

The parser needs to be slightly updated or not be applied when the action is cancelled