Clean up of non-existing instances

[rwolffgang]

I called SoundJS.play with wrong parameters, such as an unknown id, so that the SoundInstance was not fully created. Still the function HTMLAudioPlugin.cleanUp was called which in turn called SoundJS.playFinished. Since instance was never pushed to the array, indexOfis -1 and splice removes the first array element.
Below is my fix, unfortunately I'm not yet familiar enough with GitHub to create a Pull Request.

Looking for your feedback and thanks for this great library!

s.playFinished = function(instance) {
    SoundChannel.remove(instance);

    var index = this.instances.indexOf(instance);
    if (index != -1)
        this.instances.splice(index, 1);
    // Note: Keep in instance hash.
}

[lannymcnie]

I believe this fix is already pushed to Git in the NEXT build.

[rwolffgang]

You're right, I should've looked before trying to fix it. Will do next time.