virtio-net: fix wild pointer when remove virtio-net queues

The tx_bh or tx_timer will free in virtio_net_del_queue() function, when
removing virtio-net queues if the guest doesn't support multiqueue. But
it might be still referenced by virtio_net_set_status(), which needs to
be set NULL. And also the tx_waiting needs to be set zero to prevent
virtio_net_set_status() accessing tx_bh or tx_timer.

Cc: qemu-stable@nongnu.org
Signed-off-by: Yunjian Wang <wangyunjian@huawei.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>

hw/net/virtio-net.c

@@ -1522,9 +1522,12 @@ static void virtio_net_del_queue(VirtIONet *n, int index)
     if (q->tx_timer) {
         timer_del(q->tx_timer);
         timer_free(q->tx_timer);
+        q->tx_timer = NULL;
     } else {
         qemu_bh_delete(q->tx_bh);
+        q->tx_bh = NULL;
     }
+    q->tx_waiting = 0;
     virtio_del_queue(vdev, index * 2 + 1);
 }