child_process: fix data loss with readable event

This commit prevents child process stdio streams from being
automatically flushed on child process exit/close if a 'readable'
event handler has been attached at the time of exit.

Without this, child process stdio data can be lost if the process
exits quickly and a `read()` (e.g. from a 'readable' handler)
hasn't had the chance to get called yet.

Fixes: https://github.com/nodejs/node/issues/5034
PR-URL: https://github.com/nodejs/node/pull/5037


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