r24241 (#6755) introduced a bug where if the KDC sends a LastReq entry
containing an account expiry time, we send a prompter warning for
password expiry even if there was no entry containing a password
expiry time. Typically, this results in the message "Warning: Your
password will expire in less than one hour on Thu Jan 1 12:00:00
Fix this by explicitly checking for pw_exp == 0 in warn_pw_expiry()
after we've gotten past the conditional for invoking the callback.
Commit By: ghudson