dns: fix support for passing a number as second lookup() arg

This was a regression in e643fe4. @trevnorris this addresses the comment in https://github.com/joyent/node/commit/e643fe4c4b79f2683914c6435f7166fc6be9f1a7#commitcomment-7305948. I added back the removed `else` branch. I also initialized `family` to a value that will not pass validation on line 127.