pygi-struct-marshal: perform boxed copy ...
pygi-struct-marshal: perform boxed copy ...

when the cleanup marshaller will not be called, that is when
marshalling something that is not actually an argument.

Fixes #164

GNOME/pygobject,pexip/pygobject,GNOME/pygobject,GNOME/pygobject,pexip/pygobject,pexip/pygobject