platform/x86: ISST: Add a NULL check for sst_inst[]

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                    "vendor": "Linux",
                    "defaultStatus": "unaffected",
                    "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
                    "programFiles": [
                        "drivers/platform/x86/intel/speed_select_if/isst_tpmi_core.c"
                    ],
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{
                    "product": "Linux",
                    "vendor": "Linux",
                    "defaultStatus": "affected",
                    "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
                    "programFiles": [
                        "drivers/platform/x86/intel/speed_select_if/isst_tpmi_core.c"
                    ],
                    "versions": [
                        {
                            "version": "6.4",
                            "status": "affected"
                        },
                        {
                            "version": "0",
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                            "versionType": "semver"
                        },
                        {
                            "version": "6.6.160",
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                        {
                            "version": "6.12.109",
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                        {
                            "version": "6.18.50",
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                        {
                            "version": "7.3",
                            "lessThanOrEqual": "*",
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                            "versionType": "original_commit_for_fix"
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                    ]
                }