child_process: Check stderr before accessing it

If something bad happens in spawnSync, stderr might be null. Therefore,
we have to check it before using it, so we won't mask the actual
exception.

PR-URL: https://github.com/nodejs/node/pull/6877


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Robert Jefe Lindstädt <robert.lindstaedt@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>