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_is_weak_exception(krb5_principal principal, krb5_enctype etype)
{
    if (principal->name.name_string.len > 0 &&
	strcmp(principal->name.name_string.val[0], "afs") == 0 &&
	(etype == (krb5_enctype)ETYPE_DES_CBC_CRC
	 || etype == (krb5_enctype)ETYPE_DES_CBC_MD4
	 || etype == (krb5_enctype)ETYPE_DES_CBC_MD5))
	return TRUE;
    return FALSE;
}

_kdc_is_weak_exception(krb5_principal principal, krb5_enctype etype)
{
    if (principal->name.name_string.len > 0 &&
	strcmp(principal->name.name_string.val[0], "afs") == 0 &&
	(etype == (krb5_enctype)ETYPE_DES_CBC_CRC
	 || etype == (krb5_enctype)ETYPE_DES_CBC_MD4
	 || etype == (krb5_enctype)ETYPE_DES_CBC_MD5))
	return TRUE;
    return FALSE;
}
