Bug 1015042 - [Music] Use the backup duration when the audio.duration is Infinity

This might be easier to read like so:

```
if (isNaN(this.audio.duration)) {
  endTime = 0;
} else if (this.audio.duration === 'Infinity') {
  endTime = (this.audio.buffered.length > 0) ?
    this.audio.buffered.end(this.audio.buffered.length - 1) : 0;
} else {
  endTime = this.audio.duration;
}
```