tornado: add logic for when tornado runs out of steam

This is code from the original C/C++ release, modified to fix
what I perceived as a bug. After the tornado runs for 200 ticks,
there is a 0.2% chance of termination each tick.

git-svn-id: https://micropolis.googlecode.com/svn/trunk/micropolis-java@912 d9718cc8-9f43-0410-858b-315f434eb58c

src/micropolisj/engine/TornadoSprite.java

@@ -75,7 +75,11 @@ public class TornadoSprite extends Sprite
 			return;
 		}
 
-		// FIXME- the original code checks here for an ending condition
+		if (this.count == 0 && city.PRNG.nextInt(501) == 0) {
+			// early termination
+			this.frame = 0;
+			return;
+		}
 
 		destroyTile(x/16, y/16);
 	}