child_process: guard against race condition

It is possible that the internal hnadleMessage() might try to send to
a channel that has been closed. The result can be an AssertionError.
Guard against this.

Fixes: https://github.com/nodejs/node/issues/4205
PR-URL: https://github.com/nodejs/node/pull/4418


Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>