DP: Set extra_value to NULL for enum requests
    
    Some providers, notably IPA, rely on extra_value to be either a useful
    value or NULL. In enumeration, however, extra_value was random. Set
    the extra_value pointer explicitly to NULL to make it clear that it's
    not used for enumeration and also use talloc_zero as future-proof.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/2659
    
    Reviewed-by: Sumit Bose <sbose@redhat.com>