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GH-39976: [C++] Fix out-of-line data size calculation in BinaryViewBuilder::AppendArraySlice (#39994)



### Rationale for this change

Fix the bug in `BinaryViewBuilder::AppendArraySlice` that, when calculating out-of-line data size, the array is wrongly iterated.

### What changes are included in this PR?

Fix and UT.

### Are these changes tested?

UT included.

### Are there any user-facing changes?

No.

* Closes: #39976

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