Update yaf_exception.c

Strange behavior on var_dump.

```
object(Yaf_Exception_LoadFailed_Action)#13 (8) {
  ["string":"Exception":private]=>
  string(0) ""
  ["file":protected]=>
  string(29) "C:\xampp\htdocs\dev\index.php"
  ["line":protected]=>
  int(13)
  ["trace":"Exception":private]=>
  array(1) {
    [0]=>
    array(6) {
      ["file"]=>
      string(29) "C:\xampp\htdocs\dev\index.php"
      ["line"]=>
      int(13)
      ["function"]=>
      string(3) "run"
      ["class"]=>
      string(15) "Yaf_Application"
      ["type"]=>
      string(2) "->"
      ["args"]=>
      array(0) {
      }
    }
  }
  ["message":protected]=>
  string(49) "There is no method indexAction in IndexController"
  ["code":protected]=>
  int(517)
  ["previous":protected]=>
  NULL
  ["previous":"Exception":private]=>
  &UNKNOWN:0
}
```

&UNKNOWN:0 <- I would of expected this return null like it does with this change, could potentially fix the crash when using print_r on exception object.