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ARROW-11162: [C++][Parquet] Fix invalid cast on Decimal256 Parquet data

The invalid cast would occur when a variable-length bytearray field would be
decoded as Decimal256 Arrow data.

Should fix the following issue:
- https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=28750

Found by OSS-Fuzz.

Closes #9125 from pitrou/ARROW-11162-parquet-decimal256-fuzz

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>