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ARROW-12771: [C++][Compute] Fix MaybeReserve parameter in the Consume function of GroupedCountImpl

The parameter of `MaybeReserve` in `GroupedCountImpl`'s `Consume` function is incorrect.
`counts_.length()` means the buffer's byte size, it should be replaced with `num_groups_`.  Otherwise, subsequent chunked arrays with more groups are skipped as `counts_.length()` is actually `num_groups_*sizeof(int64_t)`

Closes #10309 from Crystrix/arrow-12771

Authored-by: crystrix <chenxi.li@live.com>
Signed-off-by: Krisztián Szűcs <szucs.krisztian@gmail.com>