Well, you can use the existing split_at_mut if you're worried about getting your hand-rolled unsafe implementation wrong. It's definitely reasonable if you're splitting on rows; it may or may not be what you want if you're trying to get a slice-like thing that splits on columns.

(There's no reason to avoid safe functions that happen to have unsafe implementations. For instance, the standard-library implementation of indexing a slice, as in b"hello"[3], is just a bounds check plus an unsafe dereference.)