Fix for gss_inquire_attrs_for_mech accepting NULLs
    
    As per rfc5587, gss_inquire_attrs_for_mech must accept NULL mech_attrs
    and known_mech_attrs arguments.  Up until 1.14, MIT krb5 was not ever
    passing NULLs in these fields.
    
    This fixes an interposer loop (and subsequent segmentation fault) due
    to our previous assumption that these arguments not be NULL.
    
    See also: https://tools.ietf.org/html/rfc5587#section-3.4.3
    
    Signed-off-by: Robbie Harwood <rharwood@redhat.com>
    Reviewed-by: Simo Sorce <simo@redhat.com>