Security: Avoid NULL structure pointer member dereference

This can happen in the error path when processing malformed AS
requests with a NULL client name.  Bug originally introduced on
Fri Feb 13 09:26:01 2015 +0100 in commit:

    a873e21d7c06f22943a90a41dc733ae76799390d

    kdc: base _kdc_fast_mk_error() on krb5_mk_error_ext()

Original patch by Jeffrey Altman <jaltman@secure-endpoints.com>
_kdc_set_e_text(kdc_request_t r, const char *e_text)
{
    r->e_text = e_text;
    kdc_log(r->context, r->config, 0, "%s", e_text);
}

_kdc_set_e_text(kdc_request_t r, const char *e_text)
{
    r->e_text = e_text;
    kdc_log(r->context, r->config, 0, "%s", e_text);
}
