fix for bug #2650, shipyard never stops growing

A fix for the shipyard that keeps growing. This fixes #2650

I've attached a plot (I do that a lot these days) of number of ships in Barnards Star Tranquility and Serenity shipyards, before and after bugfix, simply as a function of number of updates to the shipyard. I ran for different times. 21 of January for one, and longer for the other I think.

![bugfix](https://f.cloud.github.com/assets/619390/1918837/a623e5f8-7dc7-11e3-8dc4-044d171d18ef.png)