Relevant BUGIDs: 113238

Relevant BUGIDs: 113238

Purpose of commit: bugfix to pam_unix

Commit summary:
---------------
Fixed case where pam_unix would segfault if the app's conversation function
returned a null pointer as the password.  Since a null pointer can never be
a valid password unless the password file also has a null field (which we
already check for), we now check for a valid pointer and return PAM_AUTH_ERR
if we don't have one.