PHP notice when $cpt_post_type["rewrite_withfront"] was not set

Hi!
I had PHP notices in my sites (see the screenshot from a dev site) with the latest release version, that should be fixed with this minimal change :)

![image](https://f.cloud.github.com/assets/533162/1917168/fa91e102-7d8c-11e3-953e-e793a85b79f6.png)

And thanks for your Plugin!