iFrame breakout fix - check if `readyState` is not unknown

Fixes http://frontend.errors.gutools.co.uk/guardian/frontend/group/2367/

Fun fact, IE can return `unknown` as a `typeof` value if the ["object in question is on the other side of a COM+ bridge"](http://robertnyman.com/2005/12/21/what-is-typeof-unknown/#comment-8001). Who knew?