gobject: re-allow finalization from constructor()

Although returning NULL from constructor is strongly discouraged, some
old libraries need to keep doing it for ABI-compatibility reasons.
Given this, it's rude to forbid finalization from within
constructor(), since it would otherwise work correctly now anyway (and
the critical when returning NULL should discourage any new uses of
returning NULL from constructor()).

https://bugzilla.gnome.org/show_bug.cgi?id=661576