child_process: use stdio.fd even if it is 0

Previously, in _validateStdio we were using stdio.fd || stdio. If
stdio.fd was falsy (or 0 in the case of stdin), then the entire stdio
object would be passed which could cause a crash.

Fixes: https://github.com/nodejs/node/issues/2721
PR-URL: https://github.com/nodejs/node/pull/2727


Reviewed-By: silverwind - Roman Reiss <me@silverwind.io>
Reviewed-By: cjihrig - Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: indutny - Fedor Indutny <fedor.indutny@gmail.com>