Ticket 48206 - Crash during retro changelog trimming

Bug Description:  If the retro changelog entry is small, its possible that
                  during the trimming the reto changelog entry is not in the
                  cache after the trim, but its tries to blindly unlock it
                  from the cache, which leads to a crash.

FIx Description:  After we call the post op plugins and retrieve the entry
                  from the cache, double check that it was found.  If it
                  is not found, do not unlock it.

https://fedorahosted.org/389/ticket/48206

Reviewed by: nhosoi(Thanks!)

ldap/servers/slapd/back-ldbm/ldbm_delete.c

@@ -1257,17 +1257,24 @@ ldbm_back_delete( Slapi_PBlock *pb )
 				CACHE_RETURN(&inst->inst_cache, &e);
 			}
 		}
-		if (cache_is_in_cache(&inst->inst_cache, e)) {
-			ep_id = e->ep_id; /* Otherwise, e might have been freed. */
-			CACHE_REMOVE(&inst->inst_cache, e);
-		}
-		cache_unlock_entry(&inst->inst_cache, e);
-		CACHE_RETURN(&inst->inst_cache, &e);
-		/* 
-		 * e is unlocked and no longer in cache.
-		 * It could be freed at any moment.
+
+		/*
+		 * e could have been replaced by cache_find_id(), recheck if it's NULL
+		 * before trying to unlock it, etc.
 		 */
-		e = NULL;
+		if (e) {
+			if (cache_is_in_cache(&inst->inst_cache, e)) {
+				ep_id = e->ep_id; /* Otherwise, e might have been freed. */
+				CACHE_REMOVE(&inst->inst_cache, e);
+			}
+			cache_unlock_entry(&inst->inst_cache, e);
+			CACHE_RETURN(&inst->inst_cache, &e);
+			/*
+			 * e is unlocked and no longer in cache.
+			 * It could be freed at any moment.
+			 */
+			e = NULL;
+		}
 
 		if (entryrdn_get_switch() && ep_id) { /* subtree-rename: on */
 			/* since the op was successful, delete the tombstone dn from the dn cache */