child_process: Check stderr before accessing it

Checklist

• make -j8 test (UNIX), or vcbuild test nosign (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

child_process

Description of change

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