[pmc][cage] Fix memory leak in String.set_string_native.
[pmc][cage] Fix memory leak in String.set_string_native.

When we assign to constant string it means that we have to allocate string from
constant pool, not to allocate constant string.

git-svn-id: 6e74a02f85675cec270f5d931b0f6998666294a3@39351 d31e2699-5ff4-0310-a27c-f18f2fbe73fe

ashgti/parrot,ashgti/parrot,ashgti/parrot,ashgti/parrot,ashgti/parrot,ashgti/parrot,ashgti/parrot