fixed: calling '.start()' before overlay fades out

Fixed `Uncaught TypeError: Cannot read property 'parentNode' of null`: if user fires `.start()` before overlay entirely fades out there is the bug.

![intro-js-start-fire-bug](https://f.cloud.github.com/assets/556770/264719/9edc89be-8d98-11e2-97ef-dcad251e78d9.png)