gpgscm: Fix copying values.

* tests/gpgscm/scheme.c (copy_value): New function.
(mk_tagged_value): Use new function.
(opexe_4): Likewise for OP_SAVE_FORCED.

Occasionally, we need to copy a value from one location in the storage
to another. Scheme objects are fine. Some primitive objects,
however, require finalization, usually to free resources.

For these values, we either make a copy or acquire a reference.

Fixes e.g. a double free if a delayed expression evaluating to a
string is forced.

• Signed-off-by: Justus Winter <justus@g10code.com>