dns: lookupService() callback must be a function

lookupService() requires a callback function. This commit adds
a check to verify that the callback is actually a function.

PR-URL: https://github.com/nodejs/node/pull/8170


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Yorkie Liu <yorkiefixer@gmail.com>