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.

https://fedorahosted.org/389/ticket/47659

Reviewed by rmeggins@redhat.com (Thank you, Rich!!)