If we reach the encrypted challenge clpreauth process method without
an armor key, error out instead of crashing. This can happen if (a)
the KDC offers encrypted challenge even though the request doesn't use
FAST (the Heimdal KDC apparently does this), and (b) we fall back to
that preauth method before generating a preauthenticated request,
typically because of a prompter failure in encrypted timestamp.
Reported by Nico Williams.
(cherry picked from commit ff6aac3e018e80fa32df2e14446c6ed9595dfc3c)