child_process: `null` channel handle on close

`HandleWrap::OnClose` destroys the underlying C++ object and null's the
internal field pointer to it. Therefore there should be no references to
the wrapping JavaScript object.

`null` the process' `_channel` field right after closing it, to ensure
no crashes will happen.

Fix: https://github.com/nodejs/node/issues/2847
PR-URL: https://github.com/nodejs/node/pull/3041


Reviewed-By: Trevor Norris <trev.norris@gmail.com>