Prevent top ad slot from overflowing out of the page

Fixes this bug:

![screen shot 2014-02-18 at 15 11 12](https://f.cloud.github.com/assets/85783/2196411/eefd3d2c-98ae-11e3-80f5-87bcab9aac00.PNG)

![ ](http://1.bp.blogspot.com/-aIfeP1FkCTw/UwIdcPFZZfI/AAAAAAABGBg/00c1mK00tGE/s1600/fish-prank.gif)