Tighten string length bounds and propagate invalid jv in implode

The bound added in CVE-2026-32316 (e47e56d22) still allowed
`sizeof(jvp_string) + (currlen + len) * 2 + 1` to wrap `size_t` on
32-bit platforms. Tighten the threshold so the final allocation
fits in 32-bit `size_t`.

Also break out of `jv_string_implode` and `f_string_implode` once
`jv_string_append_codepoint` returns an invalid `jv`; otherwise the
next iteration triggers the assertion in `jvp_string_ptr` (or
invokes undefined behavior under `-DNDEBUG`).

Fixes CVE-2026-54679.

Co-authored-by: Dirk Müller <dirk@dmllr.de>