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GH-38738: [C++] Check variadic buffer counts in bounds (#38740)



### Rationale for this change

Invalid variadic buffer counts can cause allocating storage for variadic buffers to fail.

### What changes are included in this PR?

Check variadic buffer counts are valid before they are used as an allocator argument.

### Are these changes tested?

They pass with the fuzzer testcase.

### Are there any user-facing changes?

No

* Closes: #38738

Lead-authored-by: Benjamin Kietzman <bengilgit@gmail.com>
Co-authored-by: Antoine Pitrou <pitrou@free.fr>
Signed-off-by: Benjamin Kietzman <bengilgit@gmail.com>