octeontx2-af: fix NULL deref in NIX TM tree debugfs read path

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                    "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
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                        "drivers/net/ethernet/marvell/octeontx2/af/rvu_debugfs.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/net/ethernet/marvell/octeontx2/af/rvu_debugfs.c"
                    ],
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                        {
                            "version": "0",
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                        {
                            "version": "6.12.110",
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                        {
                            "version": "6.18.52",
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