The Zipfile Reference should be set to `NULL` when the `ZIPPacker` is closed not when a file in it is closed.
When calling `ZIPPacker.close` without this nothing happens because `zf` is `NULL`. (7zip could still extract the file but warned about unexpected end of file.)
This check was removed because it introduces a bug which prevents
ZIPPacker from actually adding any files, since it must be opened before
adding any files (and therefore shouldn't be NULL at the start of
`start_file`).
