Fix Empty Multipart Crossover Bug

[riesentoaster]

When other is empty, the mod calculation following the changed code throws a divided by 0 panic.

[domenukk]

How does an empty multipart entry even end up in your corpus?

[riesentoaster]

My target takes a variable number of entries, so I have a generator and mutators that may produce empty values.

And yes, that may be a bit of an abuse of Multipart, but it works (except for tiny bugs like these).