[krbdev.mit.edu #6430] If we fail to generate preauth, don't loop

[Sam Hartman via RT]

If krb5_do_preauth is called in response to a PREAUTH_REQUIRED error,
and fails to actually produce pre-authentication data, then don't try
again.

The current (1.7) code tends to try again and again and eventually
give you a toom many loops eerror.

This is made worse by treating PREAUTH_FAILED like PREAUTH_REQUIRED as
recommended by the preauth framework.
(The recommendation still seems sound, but the client should avoid looping)