vnc: fix incorrect checking condition when updating client

vs->disconnecting is set to TRUE and vs->ioc is closed, but
vs->ioc isn't set to NULL, so that the vnc_disconnect_finish()
isn't invoked when you update client in vnc_update_client()
after vnc_disconnect_start invoked. Let's using change the checking
condition to avoid resource leak.

Signed-off-by: Haibin Wang <wanghaibin.wang@huawei.com>
Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
Message-id: 1467949056-81208-1-git-send-email-arei.gonglei@huawei.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
