timers: do not expose .unref()._handle._list

The list is defunct at this point.
I believe this to be in effect a minor defect from
3eecdf9f14

PR-URL: https://github.com/nodejs/node/pull/8422
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Julien Gilli <jgilli@nodejs.org>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/timers.js

@@ -598,6 +598,9 @@ Timeout.prototype.unref = function() {
     }
 
     var handle = reuse(this);
+    if (handle) {
+      handle._list = undefined;
+    }
 
     this._handle = handle || new TimerWrap();
     this._handle.owner = this;

test/parallel/test-timers-unref-reuse-no-exposed-list.js

@@ -0,0 +1,14 @@
+'use strict';
+
+require('../common');
+const assert = require('assert');
+
+const timer1 = setTimeout(() => {}, 1).unref();
+assert.strictEqual(timer1._handle._list, undefined,
+                   'timer1._handle._list should be undefined');
+
+// Check that everything works even if the handle was not re-used.
+setTimeout(() => {}, 1);
+const timer2 = setTimeout(() => {}, 1).unref();
+assert.strictEqual(timer2._handle._list, undefined,
+                   'timer2._handle._list should be undefined');