4140552 AD Provider: Fix crash looking up forest on Samba 4

1 file Authored by sgallagh 9 years ago, Committed by jhrozek 9 years ago,
    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)