child_process: add nullptr checks after allocs

Add `CHECK_NE(·, nullptr)` after allocations made when
spawning child processes.

PR-URL: https://github.com/nodejs/node/pull/6256


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>