vnc_refresh: calling vnc_update_client might free vs

Hi all,
this patch fixes another bug in vnc_refresh: calling vnc_update_client
might cause vs to be free()ed, in this case we cannot access vs->next
right after to examine the next item on the list.
Signed-off-by: NStefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

vnc.c

@@ -2345,7 +2345,7 @@ static int vnc_refresh_server_surface(VncDisplay *vd)
 static void vnc_refresh(void *opaque)
 {
     VncDisplay *vd = opaque;
-    VncState *vs = NULL;
+    VncState *vs = NULL, *vn = NULL;
     int has_dirty = 0, rects = 0;
 
     vga_hw_update();
@@ -2354,8 +2354,10 @@ static void vnc_refresh(void *opaque)
 
     vs = vd->clients;
     while (vs != NULL) {
+        vn = vs->next;
         rects += vnc_update_client(vs, has_dirty);
-        vs = vs->next;
+        /* vs might be free()ed here */
+        vs = vn;
     }
     /* vd->timer could be NULL now if the last client disconnected,
      * in this case don't update the timer */