Remove a spot harvesters had trouble accessing in Sea & Cake map

Fixes #5939

Made that corner two cells wide. The behaviour could be seen every time without this change, with it I can't reproduce it anymore.
![screenshot from 2014-07-27 15 46 28](https://cloud.githubusercontent.com/assets/4331210/3714111/b556dec0-1595-11e4-938a-dc3e761920a0.png)