events: fix checking max listeners with `1`

Fixes #2490.

lib/events.js

@@ -115,8 +115,14 @@ EventEmitter.prototype.addListener = function(type, listener) {
     // If we've already got an array, just append.
     this._events[type].push(listener);
 
+  } else {
+    // Adding the second element, need to change to array.
+    this._events[type] = [this._events[type], listener];
+
+  }
+
   // Check for listener leak
-    if (!this._events[type].warned) {
+  if (isArray(this._events[type]) && !this._events[type].warned) {
     var m;
     if (this._maxListeners !== undefined) {
       m = this._maxListeners;
@@ -133,10 +139,6 @@ EventEmitter.prototype.addListener = function(type, listener) {
       console.trace();
     }
   }
-  } else {
-    // Adding the second element, need to change to array.
-    this._events[type] = [this._events[type], listener];
-  }
 
   return this;
 };

test/simple/test-event-emitter-check-listener-leaks.js

@@ -41,6 +41,13 @@ assert.ok(!e._events['specific'].hasOwnProperty('warned'));
 e.on('specific', function() {});
 assert.ok(e._events['specific'].warned);
 
+// only one
+e.setMaxListeners(1);
+e.on('only one', function() {});
+assert.ok(!e._events['only one'].hasOwnProperty('warned'));
+e.on('only one', function() {});
+assert.ok(e._events['only one'].hasOwnProperty('warned'));
+
 // unlimited
 e.setMaxListeners(0);
 for (var i = 0; i < 1000; i++) {