Error checking for Snapchat::$cache in getUpdates and getFriendStories

[gabrielsoldani]

Snapchat::$cache can be null or not defined in getUpdates and getFriendStories, when login fails or after a logout.

Code to reproduce:

$snap = new Snapchat('gjdrglkmdr', 'gdkrweokew'); // invalid username and password combination
$snap->getUpdates();

outputs

PHP Notice:  Undefined property: Snapchat::$cache in C:\Users\Gabriel\Documents\
GitHub\php-snapchat\src\snapchat.php on line 234

Notice: Undefined property: Snapchat::$cache in C:\Users\Gabriel\Documents\GitHu
b\php-snapchat\src\snapchat.php on line 234
PHP Fatal error:  Call to a member function get() on a non-object in C:\Users\Ga
briel\Documents\GitHub\php-snapchat\snapchat.php on line 234

Fatal error: Call to a member function get() on a non-object in C:\Users\Gabriel
\Documents\GitHub\php-snapchat\src\snapchat.php on line 234