GH-39778: [C++] Fix tail-byte access cross buffer boundary in key hash avx2

[zanmato1984]

Rationale for this change

Issue #39778 seems caused by a careless (but hard to spot) bug in key hash avx2.

What changes are included in this PR?

Fix the careless bug.

Are these changes tested?

UT included.

Are there any user-facing changes?

No.

• Closes: [C++][Acero] ASAN reports heap buffer overflow in arrow::compute::Hashing32::ProcessStripes_avx2 #39778

[zanmato1984]

cc @pitrou

[pitrou]

Thank you!

[pitrou]

@github-actions crossbow submit -g cpp

[github-actions]

Revision: 5fea9bb

Submitted crossbow builds: ursacomputing/crossbow @ actions-cfeb62c110

Task	Status
test-alpine-linux-cpp
test-build-cpp-fuzz
test-conda-cpp
test-conda-cpp-valgrind
test-cuda-cpp
test-debian-11-cpp-amd64
test-debian-11-cpp-i386
test-fedora-38-cpp
test-ubuntu-20.04-cpp
test-ubuntu-20.04-cpp-bundled
test-ubuntu-20.04-cpp-minimal-with-formats
test-ubuntu-20.04-cpp-thread-sanitizer
test-ubuntu-22.04-cpp
test-ubuntu-22.04-cpp-20
test-ubuntu-22.04-cpp-no-threading

[conbench-apache-arrow]

After merging your PR, Conbench analyzed the 6 benchmarking runs that have been run so far on merge-commit 13b2234.

There were no benchmark performance regressions. 🎉

The full Conbench report has more details. It also includes information about 202 possible false positives for unstable benchmarks that are known to sometimes produce them.

[conbench-apache-arrow]

After merging your PR, Conbench analyzed the 6 benchmarking runs that have been run so far on merge-commit 13b2234.

There were no benchmark performance regressions. 🎉

The full Conbench report has more details. It also includes information about 202 possible false positives for unstable benchmarks that are known to sometimes produce them.