AD Provider: Fix crash looking up forest on Samba 4
We were assuming that the forest had been looked up by netlogon, but
this is not available on Samba 4 domains. We need to check that the
forest is NULL and force the lookup.
Resolves:
https://fedorahosted.org/sssd/ticket/2311
Reviewed-by: Sumit Bose <sbose@redhat.com>
(cherry picked from commit 8653b547d6142b69c62daa3cd74c251b2b9aecb7)