src: add handle check to spawn_sync

This commit verifies that the child process handle is of the
correct type before trying to close it in
CloseHandlesAndDeleteLoop(). This catches the case where input
validation failed, and the child process was never actually
spawned.

Fixes: https://github.com/nodejs/node/issues/8096
Fixes: https://github.com/nodejs/node/issues/8539
Refs: https://github.com/nodejs/node/issues/9722
PR-URL: https://github.com/nodejs/node/pull/8312


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>