Added checking for Unsetting none existing session key

Under `"hybridauth / hybridauth / Hybrid / thirdparty / Facebook / facebook.php "` line 130 on method `"clearPersistentData"`

```php
 protected function clearPersistentData($key) {
   //......
    $session_var_name = $this->constructSessionVariableName($key);
    if (isset($_SESSION[$session_var_name]))
        unset($_SESSION[$session_var_name]);
  }
```

This above code is generating none existing array index warning. Due to this warning we were unable to get facebook connection work with our system.

This patch has added an extra check to verify whether `$session_var_name` key exists. 

We are using "Zend framework 2" with ScnSocialAuth and ZfcUser modules. 

Best wishes,
Rafiq