netfilter: nft_set_rbtree: fix null deref on element insertion

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                }

{
                    "product": "Linux",
                    "vendor": "Linux",
                    "defaultStatus": "affected",
                    "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
                    "programFiles": [
                        "net/netfilter/nft_set_rbtree.c"
                    ],
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                        {
                            "version": "6.4",
                            "lessThanOrEqual": "*",
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                            "versionType": "original_commit_for_fix"
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                }