gdkdisplay: Don’t call _get_instance_private(NULL)

While it’s documented as being safe, it triggers warnings from ubsan.
While we work out the best way to deal with that inside the
implementation of `G_ADD_PRIVATE` in GLib, let’s pragmatically just
short-circuit the code which triggers the warning here. This is helpful
because `gdk_display_get_debug_flags()` is called from a number of
locations within GTK, so is likely to be hit if anyone is running a UI
app under ubsan.

See https://gitlab.gnome.org/GNOME/glib/-/issues/3267#note_2033550

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

Helps: https://gitlab.gnome.org/GNOME/glib/-/issues/3267

gdk/gdkdisplay.c

@@ -2037,9 +2037,12 @@ gdk_display_get_dmabuf_formats (GdkDisplay *display)
 GdkDebugFlags
 gdk_display_get_debug_flags (GdkDisplay *display)
 {
+  if (display == NULL)
+    return _gdk_debug_flags;
+
   GdkDisplayPrivate *priv = gdk_display_get_instance_private (display);
 
-  return display ? priv->debug_flags : _gdk_debug_flags;
+  return priv->debug_flags;
 }
 
 void