gtksettings: Add a debug message if there is no default GtkSettings

Make it slightly more obvious when things are about to slide sideways
because a NULL GtkSettings has been returned to a caller. This is a
valid return value, but is rarely handled correctly.

https://bugzilla.gnome.org/show_bug.cgi?id=778382

gtk/gtksettings.c

@@ -1281,8 +1281,10 @@ gtk_settings_get_default (void)
 
   if (display)
     return gtk_settings_get_for_display (display);
-  else
-    return NULL;
+
+  g_debug ("%s() returning NULL GtkSettings object. Is a display available?",
+           G_STRFUNC);
+  return NULL;
 }
 
 static void