Non-published, multi-page post page links don't work when permalinks enabled

[Viper007Bond]

To replicate:

1. Write a post. Make it paged by putting <!--nextpage--> somewhere in the post.
   
2. Click the "Save and continue editing" button.
   
3. Click the "Preview" link.
   
4. Click the "2" link which should take you to the second page.
   

The URL in the address bar will be something like this:

http://site.com/?p=8/2/

It should be this since the post doesn't have a permalink yet:

http://site.com/?p=8&page=2

I think this'll do the trick

Originally reported by drz: #4106

Also probably applies to the 2.0.x line if we want to fix it there.

Yep. Reproduced, and the patch fixes it.

(In [5242]) Don't use fancy permalinks on draft previews. Props Viper007Bond. fixes #4117