Squashed the delete cue text token function to simplify it.

Fixes #184

This is something that I just did quickly while looking at the double free problem. This should simplify a lot of things.

I've changed it so that the delete cue text token function accepts a pointer to
a pointer to a token and sets the tokens pointer to 0 upon completion. This will stop
us from trying to free it again once it has been freed.

Please review @caitp or @rillian.