Fix reference before assignment
    
    In 'store_session_cookie', if the server does not set the session
    cookie for some reason, the 'session_cookie' variable does not get
    assigned, resulting in UnboundLocalError.  Set an initial value of
    'None'.
    
    Fixes: https://fedorahosted.org/freeipa/ticket/6636
    Reviewed-By: Martin Basti <mbasti@redhat.com>