[SSSD] [sssd PR#28][opened] PAM: call free only when memory is expected to be allocated

[fidencio]

   URL: https://github.com/SSSD/sssd/pull/28
Author: sumit-bose
 Title: #28: PAM: call free only when memory is expected to be allocated
Action: opened

PR body:
"""
This fixes a potential double-free found by Coverity which was introduced
by PR #17.
"""

To pull the PR as Git branch:
git remote add ghsssd https://github.com/SSSD/sssd
git fetch ghsssd pull/28/head:pr28
git checkout pr28