GH-49803: [C++][CI] Avoid aborting when fuzz-mutated IPC file has less batches

[pitrou]

Rationale for this change

The IPC file fuzzer can remove batches from the IPC file footer while the batches do exist physically in the embedded IPC stream.

This causes problem in differential fuzzing, because comparing the results of the IPC stream reader against the IPC file reader then fails.

This issue was found by OSS-Fuzz: https://issues.oss-fuzz.com/issues/503357759

What changes are included in this PR?

Skip differential fuzzing against the IPC stream reader if the number of batches read from the IPC file footer is not equal.

Are these changes tested?

Yes, by existing fuzz regression file.

Are there any user-facing changes?

No.

• GitHub Issue: [C++][CI] IPC file fuzzer can remove batches from IPC file #49803

[pitrou]

@github-actions crossbow submit -g cpp

[github-actions]

Revision: 2f7386e

Submitted crossbow builds: ursacomputing/crossbow @ actions-cebef960c7

Task	Status
example-cpp-minimal-build-static
example-cpp-minimal-build-static-system-dependency
example-cpp-tutorial
test-build-cpp-fuzz
test-conda-cpp
test-conda-cpp-valgrind
test-debian-13-cpp-amd64
test-debian-13-cpp-i386
test-debian-experimental-cpp-gcc-15
test-fedora-42-cpp
test-ubuntu-22.04-cpp
test-ubuntu-22.04-cpp-20
test-ubuntu-22.04-cpp-bundled
test-ubuntu-22.04-cpp-emscripten
test-ubuntu-22.04-cpp-no-threading
test-ubuntu-24.04-cpp
test-ubuntu-24.04-cpp-bundled-offline
test-ubuntu-24.04-cpp-gcc-13-bundled
test-ubuntu-24.04-cpp-gcc-14
test-ubuntu-24.04-cpp-minimal-with-formats
test-ubuntu-24.04-cpp-thread-sanitizer