Avoid alphaStar = -inf when du contains a zero

[tlbtlbtlb]

When du(i) == 0 and it's approaching a lower bound, FindAlpha can return -inf. It should only return values in [0,1]. The added check fixes it

[PatWie]

Thanks for the change. It is not obvious for me that this case can happen. Could you please provide more details when you hit this issue?