[C++] Clang 13 build fails on unused var

[asfimport]

Just a small issue. When I build with clang 13 I get the following error from a unused var warning:

/Users/willjones/Documents/arrows/arrow/cpp/src/arrow/python/numpy_to_arrow.cc:791:13: error: variable 'n' set but not used [-Werror,-Wunused-but-set-variable]
    int64_t n = 0;
            ^
/Users/willjones/Documents/arrows/arrow/cpp/src/arrow/python/numpy_to_arrow.cc:799:13: error: variable 'n' set but not used [-Werror,-Wunused-but-set-variable]
    int64_t n = 0;
            ^ 

Reporter: Will Jones / @wjones127
Assignee: Will Jones / @wjones127

PRs and other links:

• GitHub Pull Request #12359

_{Note: This issue was originally created as ARROW-15603. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
Issue resolved by pull request 12359
#12359