child_process: guard against race condition

It is possible that the internal handleMessage() 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/5153


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