dns: use length for building TXT string

Rely on the length reported by C-Ares rather than `\0`-termination
for creating the JS string for a dns TXT response.

Fixes: https://github.com/nodejs/node/issues/30688

PR-URL: https://github.com/nodejs/node/pull/30690


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>