dns: throw a TypeError in lookupService with invalid port

Previously, port was assumed to be a number and would cause an abort in
cares_wrap. This change throws a TypeError if port is not a number
before we actually hit C++.

Fixes: https://github.com/nodejs/node/issues/4837
PR-URL: https://github.com/nodejs/node/pull/4839


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Roman Klauke <romaaan.git@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>