Correctly construct Region to uphold deallocation safety invariant (backport #2062)

[mergify]

Closes #2061

To cite from the comments of the patch:

Shrinking the buffer down to the length is important to uphold a safety invariant by the dealloc method.
Passing in a differing size into the dealloc layout is considered undefined behaviour.

See: https://doc.rust-lang.org/stable/alloc/alloc/trait.GlobalAlloc.html#safety-2

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This is an automatic backport of pull request #2062 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of e0f963d has failed:

On branch mergify/bp/release/2.0/pr-2062
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	both modified:   CHANGELOG.md

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