Don't call `MEM_dupallocN` on unknown data. We don't know that this
pointer references a heap allocation, especially not an allocation made
by Blender's allocator. Even if that's the case currently, we don't want
to rely on that in the future as attribute data management gets more
flexible with implicit sharing.
Using `Array` is a simpler way to copy memory, though it does require
a bunch of boilerplate changes in the rest of the modifier.
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