Ticket #47808 - If be_txn plugin fails in ldbm_back_add, adding entry is double freed.
Bug description: Backend add ldbm_back_add frees the entry to be added,
if the add fails in the be_txn plugins. But the frontend releases the
entry when an error is returned from the backend.
Fix desxription: When the entry is freed in the backend, set NULL to
the pblock SLAPI_ADD_ENTRY to tell the frontend not to free the entry.
https://fedorahosted.org/389/ticket/47808
Reviewed by mreynolds@redhat.com (Thank you, Mark!!)
(cherry picked from commit d50f99476386b6c464884d2d16a688ecafc7fafc)