ARROW-16225: [C++][Parquet] Fix length of encryption AAD random byte generation
ARROW-16225: [C++][Parquet] Fix length of encryption AAD random byte generation

sizeof is indeed unnecessary here.
And no need to initialize the buffer to 0 either before using rand_bytes(), if the AAD is the full length of the buffer.

Closes #12935 from andersonm-ibm/rand_bytes

Authored-by: Maya Anderson <323f18c07cc722cfe3e6821e4bf8e69e9b6834d5@il.ibm.com>
Signed-off-by: Antoine Pitrou <9359a4d812173b65a3a0094cd86363e79731a3c2@python.org>

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