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MINOR: [C++] Use [] instead of exception-throwing at(i) in concatenate.cc (#35422)

### Rationale for this change

`vector::at` performs bounds checking and can throw an exception [1]. Its use is discouraged and in this specific case, the access is provably safe because array is previously resized to `buffers.size()`.

[1] https://en.cppreference.com/w/cpp/container/vector/at

### What changes are included in this PR?

Use of `operator[]` instead of `at()`.

### Are these changes tested?

By the existing concatenation tests.

Authored-by: Felipe Oliveira Carvalho <felipekde@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>