Added undefined check processPixelRGB

Phaser.BitmapData#processPixelRGB:

I was getting an undefined error when phaser tried to call setPixel32
using “result.r”.
With this additional undefined check the problem stopped and I’m able
to run this method with all my needed images.

Author: muclemente

src/gameobjects/BitmapData.js

@@ -335,7 +335,7 @@ Phaser.BitmapData.prototype = {
 
                 result = callback.call(callbackContext, pixel, tx, ty);
 
-                if (result !== false && result !== null)
+                if (result !== false && result !== null && result !== undefined)
                 {
                     this.setPixel32(tx, ty, result.r, result.g, result.b, result.a, false);
                     dirty = true;