Old bug in longobject.c + fix

[Tim Peters]

[Paul Rubin]
> Since when is there a guarantee that marshal data is portable across
> releases?  I thought the docs specifically warned that marshal data is
> NOT portable across releases.

The docs do reserve the right to make backward-incompatible changes here.
That's not the same thing as making backward-incompatible changes, let alone
making them lightly.  Search the NEWS and HISTORY files for references to
marshal, and you won't find any incompatibilities, although a while ago we
did change it to store more precision for floats.  That was both forward-
and backward- compatible.