arrow

GH-44788: [C++] Fix a couple of maybe-uninitialized warnings (#44789)

### 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

Authored-by: Raúl Cumplido <raulcumplido@gmail.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>