Fix the getCurrentSlideNumber to return 0 when it fails to find a slide

As described in #152, it's wrong to return `-1` in case we don't find a slide from a url, so it's so much better to return the first slide in such cases, it won't break anything. And if we'd want to make any assumptions on what to do on error, it is better to be done inside the `getCurrentSlideNumber`.

Also, removed all the checks on the wrong `-1` value, 'cause this function would always return something truthy.