ARROW-16238: [C++] Fix nullptr dereference when pre-buffering IPC reads

[lidavidm]

This fixes a bug when coalescing is enabled. Also, it changes the conditional so that we skip coalescing for zero-copy files instead of enabling it. Finally, it slightly refactors the tests to ensure this case is hit.

[github-actions]

https://issues.apache.org/jira/browse/ARROW-14577

[github-actions]

https://issues.apache.org/jira/browse/ARROW-16238

[westonpace]

Thanks for catching this.

[westonpace]

Minor nit: Maybe TestFileFormatGeneratorCoalesced?

[lidavidm]

Done.

[ursabot]

Benchmark runs are scheduled for baseline = 40c1226 and contender = fd5f28e. fd5f28e is a master commit associated with this PR. Results will be available as each benchmark for each run completes.
Conbench compare runs links:
[Finished ⬇️0.0% ⬆️0.0%] ec2-t3-xlarge-us-east-2
[Failed] test-mac-arm
[Finished ⬇️0.36% ⬆️0.0%] ursa-i9-9960x
[Finished ⬇️0.5% ⬆️0.04%] ursa-thinkcentre-m75q
Buildkite builds:
[Finished] <https://buildkite.com/apache-arrow/arrow-bci-benchmark-on-ec2-t3-xlarge-us-east-2/builds/609|fd5f28ec ec2-t3-xlarge-us-east-2>
[Failed] <https://buildkite.com/apache-arrow/arrow-bci-benchmark-on-test-mac-arm/builds/597|fd5f28ec test-mac-arm>
[Finished] <https://buildkite.com/apache-arrow/arrow-bci-benchmark-on-ursa-i9-9960x/builds/596|fd5f28ec ursa-i9-9960x>
[Finished] <https://buildkite.com/apache-arrow/arrow-bci-benchmark-on-ursa-thinkcentre-m75q/builds/607|fd5f28ec ursa-thinkcentre-m75q>
[Finished] <https://buildkite.com/apache-arrow/arrow-bci-benchmark-on-ec2-t3-xlarge-us-east-2/builds/608|40c1226f ec2-t3-xlarge-us-east-2>
[Failed] <https://buildkite.com/apache-arrow/arrow-bci-benchmark-on-test-mac-arm/builds/596|40c1226f test-mac-arm>
[Finished] <https://buildkite.com/apache-arrow/arrow-bci-benchmark-on-ursa-i9-9960x/builds/595|40c1226f ursa-i9-9960x>
[Finished] <https://buildkite.com/apache-arrow/arrow-bci-benchmark-on-ursa-thinkcentre-m75q/builds/606|40c1226f ursa-thinkcentre-m75q>
Supported benchmarks:
ec2-t3-xlarge-us-east-2: Supported benchmark langs: Python, R. Runs only benchmarks with cloud = True
test-mac-arm: Supported benchmark langs: C++, Python, R
ursa-i9-9960x: Supported benchmark langs: Python, R, JavaScript
ursa-thinkcentre-m75q: Supported benchmark langs: C++, Java