dns: prevent undefined values in results

When getaddrinfo linked-list results contain entries other than AF_INET
and AF_INET6, the resulting v8::Array will contain undefined values.
That's because initialization of v8::Array pre-allocates entries for all
linked-list nodes, but not all of them will be in the final results.
This commit ensures that the array only contains valid results.

PR-URL: https://github.com/nodejs/node/pull/3696


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>