LDAP: minor refactoring in auth_send() to conform to our coding style
    
    Related:
    https://pagure.io/SSSD/sssd/issue/3451
    
    A tevent _send() function should only return NULL on ENOMEM, otherwise
    it should mark the request as failed but return the req pointer. This
    was not much of an issue, before, but the next patch will add another
    function call to the auth_send call which would make error handling
    awkward.
    
    Reviewed-by: Sumit Bose <sbose@redhat.com>
    (cherry picked from commit 09091b4b60456a989ecc8c3b6f76661a14c108ba)