Add a check around an optional function

My game crashes here if I update phaser.js to a new version. It happens
when my game switches to another State object.

Author: lucbloom

src/core/StateManager.js

@@ -307,7 +307,10 @@ Phaser.StateManager.prototype = {
             //  Already got a state running?
             if (this.current)
             {
-                this.onShutDownCallback.call(this.callbackContext, this.game);
+                if (this.onShutDownCallback)
+                {
+                    this.onShutDownCallback.call(this.callbackContext, this.game);
+                }
 
                 this.game.tweens.removeAll();