src: fix intermittent SIGSEGV in resolveTxt

Fixes a SIGSEGV by making sure `txt_chunk` is not empty before setting
it on `txt_records`

Fixes: https://github.com/joyent/node/issues/9285
PR-URL: https://github.com/joyent/node/pull/9300


Reviewed-By: cjihrig - Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: jasnell - James M Snell <jasnell@gmail.com>