ares_create_query: avoid single-byte buffer overwrite

Incorrect string length calculation when passing escaped dot.

- CVE: CVE-2016-5180
- Upstream bug: https://c-ares.haxx.se/adv_20160929.html

PR-URL: https://github.com/nodejs/node/pull/8849


Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>