Fix variable initialisation in AliGenParam
Fix variable initialisation in AliGenParam

This fixes the missing decay chain problem affecting ALIROOT-7708

miranov25/AliRoot,miranov25/AliRoot,alisw/AliRoot,miranov25/AliRoot,alisw/AliRoot,coppedis/AliRoot,alisw/AliRoot,miranov25/AliRoot,coppedis/AliRoot,miranov25/AliRoot,coppedis/AliRoot,alisw/AliRoot,coppedis/AliRoot,miranov25/AliRoot,miranov25/AliRoot,alisw/AliRoot,alisw/AliRoot,alisw/AliRoot,coppedis/AliRoot,alisw/AliRoot,coppedis/AliRoot,coppedis/AliRoot,coppedis/AliRoot,miranov25/AliRoot