daemon/worker: fixed bad memory access with delayed send completion

when the timeout event was fired, it started another step in query
resolution, but there may have been pending events on previous handle
if the events on the previous handle completed, they would check the
memory of their task (which may have been non-existent) and possibly
double-close the handle

daemon/worker.c

@@ -193,7 +193,8 @@ static void qr_task_free(uv_handle_t *handle)
 static void qr_task_timeout(uv_timer_t *req)
 {
 	struct qr_task *task = req->data;
-	if (task->next_handle) {
+	if (task->next_handle) { /* Handle data may be stale when it completes */
+		task->next_handle->data = NULL;
 		qr_task_step(task, NULL);
 	}
 }
@@ -217,21 +218,27 @@ static int qr_task_on_send(struct qr_task *task, int status)
 static void on_close(uv_handle_t *handle)
 {
 	struct qr_task *task = handle->data;
-	ioreq_release(task->worker, (struct ioreq *)handle);
+	if (task) {
+		ioreq_release(task->worker, (struct ioreq *)handle);
+	} else free(handle);
 }
 
 static void on_send(uv_udp_send_t *req, int status)
 {
 	struct qr_task *task = req->data;
-	qr_task_on_send(task, status);
-	ioreq_release(task->worker, (struct ioreq *)req);
+	if (task) {
+		qr_task_on_send(task, status);
+		ioreq_release(task->worker, (struct ioreq *)req);
+	} else free(req);
 }
 
 static void on_write(uv_write_t *req, int status)
 {
 	struct qr_task *task = req->data;
-	qr_task_on_send(task, status);
-	ioreq_release(task->worker, (struct ioreq *)req);
+	if (task) {
+		qr_task_on_send(task, status);
+		ioreq_release(task->worker, (struct ioreq *)req);
+	} else free(req);
 }
 
 static int qr_task_send(struct qr_task *task, uv_handle_t *handle, struct sockaddr *addr, knot_pkt_t *pkt)