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>
