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ARROW-9928: [C++] Speed up integer parsing slightly

By exiting early out of the parsing routine when the input is exhausted, we can save a little bit a processing time.

Closes #8104 from AndrewTsao/patch-2

Lead-authored-by: Antoine Pitrou <antoine@python.org>
Co-authored-by: andi <caoshiwei@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>