Catch an invalid state early, before a potential use-after-free. This is
mainly useful for documentation purposes.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
---
 ui/console.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/ui/console.c b/ui/console.c
index 419b098c11..0cbe5033dd 100644
--- a/ui/console.c
+++ b/ui/console.c
@@ -1538,6 +1538,8 @@ void dpy_gfx_replace_surface(QemuConsole *con,
     DisplaySurface *old_surface = con->surface;
     DisplayChangeListener *dcl;
 
+    assert(old_surface != surface);
+
     con->surface = surface;
     QLIST_FOREACH(dcl, &s->listeners, next) {
         if (con != (dcl->con ? dcl->con : active_console)) {
-- 
2.12.0.191.gc5d8de91d