Ticket update.

Thanks to git-bisect and Luke Howard it appears the problem has been
identified.

The bug has been introduced with this commit:
https://github.com/krb5/krb5/commit/dc3ba26a2c9acde7ca4ed9260fdc01997511e478

The issu is in the reordering of the checksum check in
krb5_mk_req_extended()

The attached patch is a temporary workaround that shows the issue is
indeed in that reordering as I am able to pass the rpcclient test using it.
It is not final because apparently it breaks IAKRB.
A better patch should follow.