Let the game respect "no earthquakes"

Small change that should let the game respect the mapmakers decision to not have earthquakes in a level by setting the severity to 0.
Unsure if this will also stop random quakes in games were there are no pre-determined quakes left in the level file as I need a long running save.
This might fix http://code.google.com/p/corsix-th/issues/detail?id=1653&q=quake, but I could do with a save, perhaps from the original campaign that is at or above year 10 to test this on to make sure that I have not also stopped those random earthquakes.