Title: #161: pam_sss: check conversation callback
With this patch pam_sss checks if a conversation callback is available
before using it.
There is currently no test because I'm waiting for the pam-wrapper based tests
to be committed to add a test there.
To pull the PR as Git branch:
git remote add ghsssd https://github.com/SSSD/sssd
git fetch ghsssd pull/161/head:pr161
git checkout pr161