[libwnck] screen: Check for Wayland display

[Marco Trevisan <marcotrevi src gnome org>]

commit 40856004e394305c880124f3d3b5f08fc1e9a16d
Author: Olivier Crête <olivier crete collabora com>
Date:   Fri Nov 25 15:25:40 2016 -0500

    screen: Check for Wayland display
    
    The current code doesn't check hard enough and segfaults when
    starting d-feet.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=775116

 libwnck/screen.c |   11 ++++++++---
 libwnck/util.c   |    5 ++++-
 2 files changed, 12 insertions(+), 4 deletions(-)
---
diff --git a/libwnck/screen.c b/libwnck/screen.c
index 1814ec0..4977017 100644
--- a/libwnck/screen.c
+++ b/libwnck/screen.c
@@ -625,15 +625,20 @@ _wnck_screen_get_existing (int number)
  *
  * Gets the default #WnckScreen on the default display.
  *
- * Return value: (transfer none): the default #WnckScreen. The returned
- * #WnckScreen is owned by libwnck and must not be referenced or unreferenced.
+ * Return value: (transfer none) (nullable): the default #WnckScreen. The returned
+ * #WnckScreen is owned by libwnck and must not be referenced or unreferenced. This
+ * can return %NULL if not on X11.
  **/
 WnckScreen*
 wnck_screen_get_default (void)
 {
   int default_screen;
+  Display *default_display = _wnck_get_default_display ();
 
-  default_screen = DefaultScreen (_wnck_get_default_display ());
+  if (default_display == NULL)
+    return NULL;
+
+  default_screen = DefaultScreen (default_display);
 
   return wnck_screen_get (default_screen);
 }
diff --git a/libwnck/util.c b/libwnck/util.c
index 01901b7..bc235bb 100644
--- a/libwnck/util.c
+++ b/libwnck/util.c
@@ -808,9 +808,12 @@ _wnck_init (void)
 Display *
 _wnck_get_default_display (void)
 {
+  GdkDisplay *display = gdk_display_get_default ();
   /* FIXME: when we fix libwnck to not use the GDK default display, we will
    * need to fix wnckprop accordingly. */
-  return GDK_DISPLAY_XDISPLAY (gdk_display_get_default ());
+  if (!GDK_IS_X11_DISPLAY (display))
+    return NULL;
+  return GDK_DISPLAY_XDISPLAY (display);
 }
 
 /**