zlib: remove `_closed` in source

This is purely cleanup and carries no visible behavioural changes.
Up to now, `this._closed` was used in zlib.js as a
synonym of `!this._handle`. This change makes this connection
explicit and removes the `_closed` property from zlib streams,
as the previous duplication has been the cause of subtle errors
like https://github.com/nodejs/node/issues/6034.

This also makes zlib errors lead to an explicit `_close()` call
rather than waiting for garbage collection to clean up the handle,
thus returning memory resources earlier in the case of an error.

Add a getter for `_closed` so that the property remains accessible
by legacy code.

PR-URL: https://github.com/nodejs/node/pull/6574


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