knot: fix crash if timers DB not open during shutdown

src/knot/zone/zonedb-load.c

@@ -441,10 +441,12 @@ void zonedb_reload(conf_t *conf, server_t *server)
 	synchronize_rcu();
 
 	/* Sweep the timer database. */
-	int ret = zone_timers_sweep(server->timers_db, zone_exists, db_new);
-	if (ret != KNOT_EOK) {
-		log_warning("failed to clear persistent timers for removed zones (%s)",
-		            knot_strerror(ret));
+	if (server->timers_db != NULL) {
+		int ret = zone_timers_sweep(server->timers_db, zone_exists, db_new);
+		if (ret != KNOT_EOK) {
+			log_warning("failed to clear persistent timers DB (%s)",
+			            knot_strerror(ret));
+		}
 	}
 
 	/* Remove old zone DB. */

src/utils/knotd/main.c

@@ -385,7 +385,11 @@ static void write_timers(const zone_t *zone, knot_db_txn_t *txn, int *ret)
 
 static void update_timerdb(server_t *server)
 {
-	log_info("updating zone timers database");
+	if (server->timers_db == NULL) {
+		return;
+	}
+
+	log_info("updating persistent timers DB");
 
 	knot_db_txn_t txn;
 	int ret = zone_timers_write_begin(server->timers_db, &txn);
@@ -396,7 +400,8 @@ static void update_timerdb(server_t *server)
 		ret = zone_timers_write_end(&txn);
 	}
 	if (ret != KNOT_EOK) {
-		log_warning("failed to update zone timers database (%s)", knot_strerror(ret));
+		log_warning("failed to update persistent timers DB (%s)",
+		            knot_strerror(ret));
 	}
 }