agraf reported that qemu_mutex_destroy(vs->output_mutex) while failing
in vnc_disconnect_finish().
It's because vnc_worker_thread_loop() tries to unlock the mutex while
not locked. The unlocking call doesn't fail (pthread bug ?), but
the destroy call does.
Signed-off-by: Corentin Chary <corentincj@iksaif.net>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>