GC: found and fixed the last known GC bug

caused of course by simple typos/copy&paste and missing struct members.
copy now unknown (to the VM) words during gc_copy, instead of skipping them.
they will be unaccessible after the GC anyway, but we have no idea where they are
pointing to, or what the represent.
(missing volatile data, or extern .so data/ptrs might be a good guess).

example/binarytrees.pn passes now