[krbdev.mit.edu #6430] If we fail to generate preauth, don't loop

[Sam Hartman via RT]

I'm fairly sure that
    ret = krb5_libdefault_string(context, realm, KRB5_CONF_PREFERRED_PREAUTH_TYPES,
                                     &preauth_types);
                                     

is optimistic preauth..
At least Apple was assuming that code generated optimistic preauth.