Don't panic! If the cracks are small, just proceed with rolling up your roll and pop it in the fridge. When you add your chocolate the cracks will be hidden! For larger cracks I still recommend proceeding and filling the cake, then you can wrap the entire roll tightly in plastic wrap, pulling the wrap tightly to reshape it if the cracks are so bad that it's no longer round. Place the re-shaped roll in the fridge until completely chilled then remove from the wrap and cover with chocolate.