[gtk+] GdkWaylandWindow: Clear export user data when used

[Matthias Clasen <matthiasc src gnome org>]

commit 6d7749898ddafa98381b9d412f01bfde91e84c42
Author: Jonas Ådahl <jadahl gmail com>
Date:   Fri May 5 19:07:04 2017 +0800

    GdkWaylandWindow: Clear export user data when used
    
    It was only cleared when unexported, but we could just as well clear it
    when its used too.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=782109

 gdk/wayland/gdkwindow-wayland.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)
---
diff --git a/gdk/wayland/gdkwindow-wayland.c b/gdk/wayland/gdkwindow-wayland.c
index 82835b5..94fb116 100644
--- a/gdk/wayland/gdkwindow-wayland.c
+++ b/gdk/wayland/gdkwindow-wayland.c
@@ -3967,7 +3967,8 @@ xdg_exported_handle (void                    *data,
   GdkWindowImplWayland *impl = GDK_WINDOW_IMPL_WAYLAND (window->impl);
 
   impl->exported.callback (window, handle, impl->exported.user_data);
-  impl->exported.user_data = NULL;
+  g_clear_pointer (&impl->exported.user_data,
+                   impl->exported.destroy_func);
 }
 
 static const struct zxdg_exported_v1_listener xdg_exported_listener = {