Copy `kk` and `vk` before passing to jv_object_get(), which consumes
its arguments. Without copies, subsequent loop iterations read freed
memory (CWE-416) and post-loop jv_free() double-frees (CWE-415).
This bug was introduced in `b279713e` when array processing was hoisted
out of
the loop into locals without accounting for jv_object_get() semantics.
Fixes GHSA-gf4g-95wj-4q4r.
