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>
is_default_salt_p(const krb5_salt *default_salt, const Key *key)
{
    if (key->salt == NULL)
	return TRUE;
    if (default_salt->salttype != key->salt->type)
	return FALSE;
    if (krb5_data_cmp(&default_salt->saltvalue, &key->salt->salt))
	return FALSE;
    return TRUE;
}

is_default_salt_p(const krb5_salt *default_salt, const Key *key)
{
    if (key->salt == NULL)
	return TRUE;
    if (default_salt->salttype != key->salt->type)
	return FALSE;
    if (krb5_data_cmp(&default_salt->saltvalue, &key->salt->salt))
	return FALSE;
    return TRUE;
}
