GH-44788: [C++] Fix a couple of maybe-uninitialized warnings

[raulcd]

Rationale for this change

I was getting some -Wmaybe-uninitialized warnings locally when building, see original issue for log.

What changes are included in this PR?

A couple of minor changes to avoid the warnings, initializing a variable and removing an unnecessary one.

Are these changes tested?

Code will be tested via CI and I can confirm I can compile without warnings now:

$ cmake --build arrow/cpp/build --target install -- -j 8
[411/412] Install the project...
-- Install configuration: "RELEASE"

Are there any user-facing changes?

No

• GitHub Issue: #44788

[github-actions[bot]]

:warning: GitHub issue #44788 has been automatically assigned in GitHub to PR creator.

[pitrou]

@raulcd Unrelated to this PR, but you probably want to compile in Debug mode when developing Arrow C++.

[raulcd]

@raulcd Unrelated to this PR, but you probably want to compile in Debug mode when developing Arrow C++.

Thanks for sharing! I really thought I was using the ninja-debug-python preset but it seems I forgot to update when copy&pasting the documentation from which defaults to ninja-release-python: https://github.com/apache/arrow/blob/27bbd593625122a4a25d9471c8aaf5df54a6dcf9/docs/source/developers/python.rst?plain=1#L305-L307 Maybe we should default the commands to build on debug on the development docs and mention that it can be changed to release instead of the current?

[pitrou]

Maybe we should default the commands to build on debug on the development docs and mention that it can be changed to release instead of the current?

Ah, probably indeed.