vnc: qemu can die if the client is disconnected while updating screen

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>

ui/vnc-jobs-async.c

@@ -227,6 +227,10 @@ static int vnc_worker_thread_loop(VncJobQueue *queue)
 
         if (job->vs->csock == -1) {
             vnc_unlock_display(job->vs->vd);
+            /* output mutex must be locked before going to
+             * disconnected:
+             */
+            vnc_lock_output(job->vs);
             goto disconnected;
         }