speakup: keyhelp: guard letter_offsets possible out-of-range indexing

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                    "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
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                        "drivers/accessibility/speakup/keyhelp.c"
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{
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                    "vendor": "Linux",
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                    "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
                    "programFiles": [
                        "drivers/accessibility/speakup/keyhelp.c"
                    ],
                    "versions": [
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                            "status": "affected"
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                        {
                            "version": "0",
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                            "lessThanOrEqual": "*",
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                            "versionType": "original_commit_for_fix"
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                    ]
                }