[C++] Maybe uninitialized warnings

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Building CXX object src/arrow/CMakeFiles/arrow_objlib.dir/compute/exec/key_hash.cc.o
/home/keith/git/arrow/cpp/src/arrow/compute/exec/key_hash.cc: In static member function 'static void arrow::compute::Hashing::hash_varlen_helper(uint32_t, const uint8_t*, uint32_t*)':
/home/keith/git/arrow/cpp/src/arrow/compute/exec/key_hash.cc:202:16: warning: 'last_stripe' may be used uninitialized in this function [-Wmaybe-uninitialized]
  202 |       uint32_t lane = reinterpret_cast<const uint32_t*>(last_stripe)[j];
      |                ^~~~

Building CXX object src/arrow/CMakeFiles/arrow_objlib.dir/tensor.cc.o
/home/keith/git/arrow/cpp/src/arrow/tensor.cc: In member function 'arrow::Result<long int> arrow::Tensor::CountNonZero() const':
/home/keith/git/arrow/cpp/src/arrow/tensor.cc:337:18: warning: '*((void*)& counter +8)' may be used uninitialized in this function [-Wmaybe-uninitialized]
  337 |   NonZeroCounter counter(*this);

Reporter: Keith Kraus / @kkraus14
Assignee: Keith Kraus / @kkraus14

PRs and other links:

• GitHub Pull Request #10907

_{Note: This issue was originally created as ARROW-13600. Please see the migration documentation for further details.}

[asfimport]

Yibo Cai / @cyb70289:
Issue resolved by pull request 10907
#10907