Changed to safely call  SWF functions for disposing videojs

The media-source plugin calls SWF functions and I got an error when I have called videojs.dispose() as below. I confirmed on IE10, IE11 and Chrome35, but it didn't occurred on Firefox31 somehow. I think it should be call after checking those function's existence.

```js
TypeError: undefined is not a function
    at abort (http://path/to/videojs-media-sources.js:217:26)
    at videojs.Hls.dispose (http://path/to/videojs.hls.js:247:23)
    at vjs.Player.dispose (http://path/to/video.dev.js:3448:30)
    ...
```