lib: fix undefined timeout regression

63644dd introduced a regression caused by everyone's favourite
JavaScript feature: undefined < 0 === undefined >= 0.

Add a case to the existing tests to cover this scenario and then add
the check for undefined that makes the test pass.

Author: rmg

lib/timers.js

@@ -30,7 +30,7 @@ var lists = {};
 // with them.
 exports.active = function(item) {
   const msecs = item._idleTimeout;
-  if (msecs < 0) return;
+  if (msecs < 0 || msecs === undefined) return;
 
   item._idleStart = Timer.now();
 

test/parallel/test-timers-active.js

@@ -22,7 +22,8 @@ legitTimers.forEach(function(legit) {
 
 // active() should not create a timer for these
 var bogusTimers = [
-  { _idleTimeout: -1 }
+  { _idleTimeout: -1 },
+  { _idleTimeout: undefined }
 ];
 
 bogusTimers.forEach(function(bogus) {