Get correct fin span when last point in shape is loewr than first point.

Note that the lowest point on the fin is guaranteed to be either the first point which is at (0, 0) or the last point, which will have a y coordinate of 0 if on a body tube, greater than 0 if on an increasing transition, or less than 0 on a decreasing transition (boattail).

core/src/net/sf/openrocket/rocketcomponent/FreeformFinSet.java

@@ -296,7 +296,8 @@ public class FreeformFinSet extends FinSet {
 			if (c.y > max)
 				max = c.y;
 		}
-		return max;
+
+		return max - Math.min(points.get(points.size() - 1).y, 0);
 	}
 	
 	@Override