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ARROW-14898: [C++][Compute] Fix crash of out-of-bounds memory accessing in key_hash if a key is smaller than int64

I encountered a crash when executing GroupBy on specific data. The code and data to reproduce the crash can be found in the related JIRA ticket https://issues.apache.org/jira/browse/ARROW-14898

I think the root cause is the tail process in `Hashing::hash_varlen` of `key_hash.cc`.
The steps of related code are as follows:
1. `Hashing::hash_varlen` calls `helper_tail` for the tail part of the key
2. `helper_tail` calls `util::SafeLoadAs` to load 8 bytes of data from the key
3. `util::SafeLoadAs` calls `std::memcpy` to copy 8 bytes of data from the key

If the key is less than 8 bytes, the `std::memcpy` still copies 8 bytes which may access illegal memory.

This PR adds a `length` parameter to those functions to copy just the size of the key for the tail.
I'm not sure how to add a UT to test it, as it only happens on my specific data.
The AVX2 code also has this crash, the fix for AVX2 is not in this PR.

Closes #11789 from Crystrix/arrow-14898

Authored-by: crystrix <chenxi.li@live.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>