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ARROW-15360: [Python] Check slice bounds in Buffer.slice()

Calling `Buffer.slice()` with invalid arguments could return a Buffer referencing out-of-bounds memory.
With this change, an IndexError is raised instead.

Note that `Buffer.__getitem__()` is unaffected, as it applies Python slice semantics: indices out of bounds are automatically clamped.

Closes #12182 from pitrou/ARROW-15360-buffer-slice

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