console: fix dpy_gfx_replace_surface assert

virtio-gpu can trigger the assert added by commit "6905b934 console:
add same surface replace pre-condition" in multihead setups (where
surface can be NULL for secondary displays).  Allow surface being NULL.

Fixes: 6905b934Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>
Reviewed-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 20170906142109.2685-1-kraxel@redhat.com

ui/console.c

@@ -1540,7 +1540,7 @@ void dpy_gfx_replace_surface(QemuConsole *con,
     DisplaySurface *old_surface = con->surface;
     DisplayChangeListener *dcl;
 
-    assert(old_surface != surface);
+    assert(old_surface != surface || surface == NULL);
 
     con->surface = surface;
     QLIST_FOREACH(dcl, &s->listeners, next) {