bpo-29990: Range checking in GB18030 decoder

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[the-knights-who-say-ni]

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[mention-bot]

@animalize, thanks for your PR! By analyzing the history of the files in this pull request, we identified @hyeshik, @bitdancer and @Yhg1s to be potential reviewers.

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I signed CLA, and associated GitHub username in b.p.o. But the label CLA not signed still exist.