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GH-40652: [C++] Enlarge dest buffer according to dest offset for `CopyBitmap` benchmark (#40769)



### Rationale for this change

`CopyBitmap` benchmark doesn't make dest buffer large enough wrt. the possible dest offset, causing memory issues like when accessing the trailing bytes. 

### What changes are included in this PR?

Make the dest buffer wrt. dest offset.

### Are these changes tested?

The fix itself is test (benchmark).

### Are there any user-facing changes?

None.

* GitHub Issue: #40652

Authored-by: Ruoxi Sun <zanmato1984@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>