vnc_refresh: return if vd->timer is NULL

Hi all,
calling vnc_update_client in vnc_refresh might have the unlikely side
effect of setting vd->timer = NULL, if the last vnc client disconnected.
In this case we have to return from vnc_refresh without updating the
timer, otherwise we cause a segfault.
Signed-off-by: NStefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

vnc.c

@@ -2305,6 +2305,10 @@ static void vnc_refresh(void *opaque)
         rects += vnc_update_client(vs, has_dirty);
         vs = vs->next;
     }
+    /* vd->timer could be NULL now if the last client disconnected,
+     * in this case don't update the timer */
+    if (vd->timer == NULL)
+        return;
 
     if (has_dirty && rects) {
         vd->timer_interval /= 2;