Undefined post data causing server to crash

The issue occurs when the page is not the index, static (I assume that's what /page/ refers to), or a post. The default condition assumes there is post data associated with the page to be rendered, but if there isn't, the server chokes.

    /home/action/core/server/helpers/index.js:239 
    title = this.post.title; 
                     ^ 
    TypeError: Cannot read property ‘title’ of undefined 

---

Personally I have only ran into this issue hacking around, but this was an issue brought up on the forums: [Crash when i refer to another domain](https://en.ghost.org/forum/bugs-suggestions/1761-crash-when-i-refer-to-another-domain).

This commit fixes this issue by checking if the post data exists before trying to dereference it.