ITS#4572 don't unlock non-existent locks

servers/slapd/back-bdb/cache.c

@@ -146,7 +146,7 @@ bdb_cache_entry_db_unlock ( DB_ENV *env, DB_LOCK *lock )
 #else
 	int rc;
 
-	if ( !lock ) return 0;
+	if ( !lock || lock->mode == DB_LOCK_NG ) return 0;
 
 	rc = LOCK_PUT ( env, lock );
 	return rc;

servers/slapd/back-bdb/dn2entry.c

@@ -56,8 +56,11 @@ bdb_dn2entry(
 				rc2 = bdb_cache_find_id( op, tid, ei->bei_id,
 					&ei, 1, locker, lock );
 				if ( rc2 ) rc = rc2;
-			} else if ( ei )
+			} else if ( ei ) {
 				bdb_cache_entryinfo_unlock( ei );
+				memset( lock, 0, sizeof( *lock ));
+				lock->mode = DB_LOCK_NG;
+			}
 		} else if ( ei ) {
 			bdb_cache_entryinfo_unlock( ei );
 		}