die/exit from within exported PHP functions causes segfault

If JS context calls a PHP function that triggers either `die` or `exit`, the engine segfaults.

Example:

``` php
<?php
$v8 = new V8Js();
$v8->foo = function() { echo "Hallo\n"; die("blar ...\n"); };
$v8->executeString('PHP.foo();');
```
