Fix spurious password expiry warning

Fix spurious password expiry warning

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
1970".

Fix this by explicitly checking for pw_exp == 0 in warn_pw_expiry()
after we've gotten past the conditional for invoking the callback.

ticket: 7098
target_version: 1.9.4
tags: pullup

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@25730 dc483132-0cff-0310-8789-dd5450dbe970