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: [C++] Potentially uninitialized variable warnings  #44788

[github-actions]

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