Ticket #47659 - ldbm_usn_init: Valgrind reports Invalid read / SIGSEGV
Bug description: A suffix mapping tree could exist without the corresponding
backend. The existing code did not properly check the backend returned from
slapi_mapping_tree_find_backend_for_sdn. When NULL backend is returned, it
triggers the NULL pointer dereference.
Fix description: This patch added a NULL backend check to usn_get_last_usn,
and moved a logging to the if clause where the backend is not NULL.
Reviewed by firstname.lastname@example.org (Thank you, Rich!!)
(cherry picked from commit a6f66e7fcbd5d17d975cc2ac65806d7c64571254)