debugdeltachain: avoid division by zero when a chain is empty The two ratios chainratio and extraratio are computed using dividers that may be zero when the file is empty. As the denominators are integers, the limit of the ratio "just before zero" is the numerator value itself. If the numerator itself is zero, the ratio value is still meaningful: in both cases, a "good" value is a low ratio, and a size of zero is the optimal case.