`VMABuffer::set_data` may allow out-of-bounds writes from safe code

`VMABuffer::set_data` was a publicly accessible safe function. It accepted an arbitrary `offset` and a data slice, then used the offset in unsafe pointer arithmetic before copying the slice into a mapped allocation.

Affected versions did not check that the requested write range fit within the allocation before calling `ptr.add(offset)` and `copy_from_nonoverlapping`. Safe Rust code could therefore trigger an out-of-bounds write by passing an offset outside the mapped allocation.

This makes the safe API unsound, since callers can trigger undefined behavior without using `unsafe`.

Version `0.4.0` added a bounds check before performing the pointer arithmetic and copy.