mptcp: fix NULL pointer in can_accept_new_subflow

{
                    "product": "Linux",
                    "vendor": "Linux",
                    "defaultStatus": "unaffected",
                    "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
                    "programFiles": [
                        "net/mptcp/subflow.c"
                    ],
                    "versions": [
                        {
                            "version": "9466a1ccebbe54ac57fb8a89c2b4b854826546a8",
                            "lessThan": "8cf7fef1bb2ffea7792bcbf71ca00216cecc725d",
                            "status": "affected",
                            "versionType": "git"
                        },
                        {
                            "version": "9466a1ccebbe54ac57fb8a89c2b4b854826546a8",
                            "lessThan": "b3088bd2a6790c8efff139d86d7a9d0b1305977b",
                            "status": "affected",
                            "versionType": "git"
                        },
                        {
                            "version": "9466a1ccebbe54ac57fb8a89c2b4b854826546a8",
                            "lessThan": "855bf0aacd51fced11ea9aa0d5101ee0febaeadb",
                            "status": "affected",
                            "versionType": "git"
                        },
                        {
                            "version": "9466a1ccebbe54ac57fb8a89c2b4b854826546a8",
                            "lessThan": "7f9ae060ed64aef8f174c5f1ea513825b1be9af1",
                            "status": "affected",
                            "versionType": "git"
                        },
                        {
                            "version": "9466a1ccebbe54ac57fb8a89c2b4b854826546a8",
                            "lessThan": "dc81e41a307df523072186b241fa8244fecd7803",
                            "status": "affected",
                            "versionType": "git"
                        },
                        {
                            "version": "9466a1ccebbe54ac57fb8a89c2b4b854826546a8",
                            "lessThan": "efd58a8dd9e7a709a90ee486a4247c923d27296f",
                            "status": "affected",
                            "versionType": "git"
                        },
                        {
                            "version": "9466a1ccebbe54ac57fb8a89c2b4b854826546a8",
                            "lessThan": "4b2649b9717678aeb097893cc49f59311a1ecab0",
                            "status": "affected",
                            "versionType": "git"
                        },
                        {
                            "version": "9466a1ccebbe54ac57fb8a89c2b4b854826546a8",
                            "lessThan": "443041deb5ef6a1289a99ed95015ec7442f141dc",
                            "status": "affected",
                            "versionType": "git"
                        }
                    ]
                }

{
                    "product": "Linux",
                    "vendor": "Linux",
                    "defaultStatus": "affected",
                    "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
                    "programFiles": [
                        "net/mptcp/subflow.c"
                    ],
                    "versions": [
                        {
                            "version": "5.9",
                            "status": "affected"
                        },
                        {
                            "version": "0",
                            "lessThan": "5.9",
                            "status": "unaffected",
                            "versionType": "semver"
                        },
                        {
                            "version": "5.10.237",
                            "lessThanOrEqual": "5.10.*",
                            "status": "unaffected",
                            "versionType": "semver"
                        },
                        {
                            "version": "5.15.181",
                            "lessThanOrEqual": "5.15.*",
                            "status": "unaffected",
                            "versionType": "semver"
                        },
                        {
                            "version": "6.1.135",
                            "lessThanOrEqual": "6.1.*",
                            "status": "unaffected",
                            "versionType": "semver"
                        },
                        {
                            "version": "6.6.88",
                            "lessThanOrEqual": "6.6.*",
                            "status": "unaffected",
                            "versionType": "semver"
                        },
                        {
                            "version": "6.12.24",
                            "lessThanOrEqual": "6.12.*",
                            "status": "unaffected",
                            "versionType": "semver"
                        },
                        {
                            "version": "6.13.12",
                            "lessThanOrEqual": "6.13.*",
                            "status": "unaffected",
                            "versionType": "semver"
                        },
                        {
                            "version": "6.14.3",
                            "lessThanOrEqual": "6.14.*",
                            "status": "unaffected",
                            "versionType": "semver"
                        },
                        {
                            "version": "6.15",
                            "lessThanOrEqual": "*",
                            "status": "unaffected",
                            "versionType": "original_commit_for_fix"
                        }
                    ]
                }