avoid accessing deleted race objects

see
http://www.forum.birth-of-the-empires.de/viewtopic.php?f=68&t=2576&sid=0c0d700bb87c3e294eada28e41a942f1
When a game is loaded, race controller deletes races that aren't needed.
Is the load aborted and another game loaded (or a new one is started)
these races are needed again however.