g_clear_{s,}list() in GLib 2.63.3

On the topic of new APIs in GLib, https://gitlab.gnome.org/GNOME/glib/commit/58ba7d78fbd7ecb4c0df2dc7e251627ebbffb9d5 is now a thing (whenever 2.63.3 sees the light of day).

Nothing super exciting, but it will scratch my own itch in refactoring some code later, since we do lots of (or variants of)

if (NULL != bar)
    foo_free (bar);
    bar = NULL;

That also includes lists, for which there is no sugar in case you want to also free the elements. With the change, you will be able to simply write

g_clear_list (&slist, NULL);    // does g_list_free()
g_clear_slist (&slist, g_free); // does g_slist_free_full()