ARROW-13104: [C++] Fix unsafe cast in ByteStreamSplit implementation
ARROW-13104: [C++] Fix unsafe cast in ByteStreamSplit implementation

Closes #10596 from pitrou/ARROW-13104-unsafe-cast

Authored-by: Antoine Pitrou <9359a4d812173b65a3a0094cd86363e79731a3c2@python.org>
Signed-off-by: Antoine Pitrou <9359a4d812173b65a3a0094cd86363e79731a3c2@python.org>

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