Fix for becomeForward on 64 bit images

There is a bug in becomeForward on 64 bit images that results in a
VM crash if, for example, one attempts to load VMMaker. The bug
report and the fix are on Mantis:
        http://bugs.impara.de/view.php?id=5401

This was just a small editing oversight, but the bug has been
present since the original dist3 release. I'm happy to report that
it is now possible to load the latest VMMaker into a 64 bit image.

I think this patch would be a good candidate to include in the
next VMMaker release ;-)

Dave