timers: cleanup interval handling

Uses `null` as the false-y value for `_repeat` as like other properties.
Removes un-reachable statement in setInterval’s `wrapper()`.

PR-URL: #1272
Reviewed-by: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

lib/timers.js

@@ -272,8 +272,6 @@ exports.setInterval = function(callback, repeat) {
 
   function wrapper() {
     timer._repeat.call(this);
-    // If callback called clearInterval().
-    if (timer._repeat === null) return;
     // If timer is unref'd (or was - it's permanently removed from the list.)
     if (this._handle) {
       this._handle.start(repeat, 0);
@@ -287,7 +285,7 @@ exports.setInterval = function(callback, repeat) {
 
 exports.clearInterval = function(timer) {
   if (timer && timer._repeat) {
-    timer._repeat = false;
+    timer._repeat = null;
     clearTimeout(timer);
   }
 };
@@ -300,7 +298,7 @@ const Timeout = function(after) {
   this._idleNext = this;
   this._idleStart = null;
   this._onTimeout = null;
-  this._repeat = false;
+  this._repeat = null;
 };
 
 Timeout.prototype.unref = function() {