pam_sss: check conversation callback
With this patch pam_sss checks if a conversation callback is available
before using it.
Resolves https://fedorahosted.org/sssd/ticket/3296
Reviewed-by: Pavel Březina <pbrezina@redhat.com>
(cherry picked from commit 0965a77c4ff0b358d24582955cb7ae375ebaa0d2)
(cherry picked from commit ba8e3f2850e5a328bc3e732b471280fc4fa49c53)