timers: fix subsequent enroll calls not working

A bug was introduced in nodejs#17704 which meant that subsequent
calls to enroll would unset the new _idleTimeout and the
enrolled object could never again function as a timer.

lib/timers.js

@@ -353,13 +353,14 @@ exports.unenroll = util.deprecate(unenroll,
 // This function does not start the timer, see `active()`.
 // Using existing objects as timers slightly reduces object overhead.
 function enroll(item, msecs) {
-  item._idleTimeout = validateTimerDuration(msecs);
+  msecs = validateTimerDuration(msecs);
 
   // if this item was already in a list somewhere
   // then we should unenroll it from that
   if (item._idleNext) unenroll(item);
 
   L.init(item);
+  item._idleTimeout = msecs;
 }
 
 exports.enroll = util.deprecate(enroll,

test/parallel/test-timers-enroll-second-time.js

@@ -0,0 +1,16 @@
+'use strict';
+
+const common = require('../common');
+
+const assert = require('assert');
+const timers = require('timers');
+
+let didCall = false;
+const enrollObj = {
+  _onTimeout: common.mustCall(() => assert(didCall)),
+};
+
+timers.enroll(enrollObj, 1);
+timers.enroll(enrollObj, 10);
+timers.active(enrollObj);
+setTimeout(common.mustCall(() => { didCall = true; }), 1);