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GH-38833: [C++] Avoid hash_mean overflow (#39349)



### Rationale for this change

hash_mean overflows if the sum of a group is larger than uint64 max.

### What changes are included in this PR?

Save the intermediate sum as double to avoid overflow

### Are these changes tested?

yes

### Are there any user-facing changes?
 no

* Closes: #38833

Authored-by: Jin Shang <shangjin1997@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>