tap: clear vhost_net backend on cleanup

Frontends calling tap_get_vhost_net get an invalid pointer after the
peer backend has been deleted. Jason Wang <jasowang@redhat.com> reports
this leading to a crash in ack_features when we remove the vhost-net
bakend of a virtio nic.

The fix is simply to clear the backend pointer.
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

net/tap.c

@@ -279,6 +279,7 @@ static void tap_cleanup(VLANClientState *nc)
 
     if (s->vhost_net) {
         vhost_net_cleanup(s->vhost_net);
+        s->vhost_net = NULL;
     }
 
     qemu_purge_queued_packets(nc);