Fixes issue with custom Particle from BitmapData

 * Added null/undefined check for this._frameData
 * Fixes issue presented when providing a ParticleEmitter with a
 custom Particle class that was introduced sometime after version
 2.0.7 (for examples see: http://examples.phaser.io/_site/view_full.html?d=particles&f=particle+class.js&t=particle%20class)