gobject: add internal g_destroy_notify_assert_not_reached() helper

This is a GDestoryNotify that asserts not to be called. Obviously, this
is only for testing and asserting (defensive programming).

It is private API inside "libgobject-2.0.so.0". If unused, we rely that
the linker can strip it out.

gobject/gtype-private.h

@@ -99,6 +99,8 @@ gboolean    _g_object_has_signal_handler     (GObject     *object);
 void        _g_object_set_has_signal_handler (GObject     *object,
                                               guint        signal_id);
 
+void g_destroy_notify_assert_not_reached (gpointer object);
+
 /**
  * _G_DEFINE_TYPE_EXTENDED_WITH_PRELUDE:
  *

gobject/gtype.c

@@ -4848,3 +4848,9 @@ g_type_ensure (GType type)
   if (G_UNLIKELY (type == (GType)-1))
     g_error ("can't happen");
 }
+
+void
+g_destroy_notify_assert_not_reached (gpointer object)
+{
+  g_return_if_reached ();
+}