vnc: move assert in vnc_worker_thread_loop

job may be NULL if queue->exit is true. Check
it before dereference job.

Fixes: f31f9c1 ("vnc: add magic cookie to VncState")
Signed-off-by: Anastasia Belova <abelova@astralinux.ru>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
(cherry picked from commit bdfca8a)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

ui/vnc-jobs.c

@@ -250,12 +250,13 @@ static int vnc_worker_thread_loop(VncJobQueue *queue)
     /* Here job can only be NULL if queue->exit is true */
     job = QTAILQ_FIRST(&queue->jobs);
     vnc_unlock_queue(queue);
-    assert(job->vs->magic == VNC_MAGIC);
 
     if (queue->exit) {
         return -1;
     }
 
+    assert(job->vs->magic == VNC_MAGIC);
+
     vnc_lock_output(job->vs);
     if (job->vs->ioc == NULL || job->vs->abort == true) {
         vnc_unlock_output(job->vs);