Merge pull request cocos2d#2491 from pandamicro/develop

cocos2d/cocos2d-js#1108: Fix audioEngine absent issue

Author: pandamicro

CCBoot.js

@@ -2052,7 +2052,7 @@ cc.game = /** @lends cc.game# */{
     restart: function () {
         cc.director.popToSceneStackLevel(0);
         // Clean up audio
-        cc.audioEngine.end();
+        cc.audioEngine && cc.audioEngine.end();
 
         cc.game.onStart();
     },