Checking for malformed signed request

In `FacebookSession.php` file, `private static function parseSignedRequest($signedRequest, $state)` method is missing `else` clause for `if` statement:
```php
if (strpos($signedRequest, '.') !== false) {
```
It can result in sending requests to Graph API without access token because `parseSignedRequest` method will return `null` (because the `return` keyword is omitted) when malformed signed request is passed.

Added `else` statement.