deps: c-ares, avoid single-byte buffer overwrite

This is a manual backport of https://github.com/nodejs/node/pull/8849 because we're jumping back from 1.10 to 1.9 here. It's pretty awkward because there's no max_udp_size argument in this old form. Tests pass locally but I don't have the highest degree of confidence that I've got this 100% so I'd appreciate some more expert eyes please.

Maybe one of @indutny, @bnoordhuis, @addaleax might be able to grok this better than I?