child_process: add safety checks on stdio access

When a child process is spawned, there is no guarantee that stdout
and stderr will be created successfully. This commit adds checks
before attempting to access the streams.

PR-URL: https://github.com/nodejs/node/pull/3799


Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: James M Snell <jasnell@gmail.com>