Fix UB in Subbuffer::reinterpret

[marc0246]

This could have lead to UB if the stars aligned just right. It only concerns reinterpretation of a lesser-aligned type to a custom DST that has a higher-aligned head plus a lesser-aligned tail:

#[derive(BufferContents)]
#[repr(C)]
struct Example {
    head: u32,
    tail: [u8],
}

let s: Subbuffer<[u8; 5]>;

s.reinterpret::<Example>().write();

The last line is UB because there could be another subbuffer or other allocation right after the given subbuffer, even though the &mut Example above has provenance over 3 bytes past the subbuffer, which could result in an aliasing mutable reference.

At least Subbuffer::reinterpret hasn't been released yet. :)