cluster: don't send messages if no IPC channel

Avoid sending messages if the IPC channel is already disconnected. It
avoids undesired errors when calling `process.disconnect` when there are
still pending IPC messages.

PR-URL: https://github.com/nodejs/node/pull/7132


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>