nbd: null check for nla_nest_start

{
                    "product": "Linux",
                    "vendor": "Linux",
                    "defaultStatus": "unaffected",
                    "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
                    "programFiles": [
                        "drivers/block/nbd.c"
                    ],
                    "versions": [
                        {
                            "version": "47d902b90a32a42a3d33aef3a02170fc6f70aa23",
                            "lessThan": "44214d744be32a4769faebba764510888f1eb19e",
                            "status": "affected",
                            "versionType": "git"
                        },
                        {
                            "version": "47d902b90a32a42a3d33aef3a02170fc6f70aa23",
                            "lessThan": "4af837db0fd3679fabc7b7758397090b0c06dced",
                            "status": "affected",
                            "versionType": "git"
                        },
                        {
                            "version": "47d902b90a32a42a3d33aef3a02170fc6f70aa23",
                            "lessThan": "98e60b538e66c90b9a856828c71d4e975ebfa797",
                            "status": "affected",
                            "versionType": "git"
                        },
                        {
                            "version": "47d902b90a32a42a3d33aef3a02170fc6f70aa23",
                            "lessThan": "96436365e5d80d0106ea785a4f80a58e7c9edff8",
                            "status": "affected",
                            "versionType": "git"
                        },
                        {
                            "version": "47d902b90a32a42a3d33aef3a02170fc6f70aa23",
                            "lessThan": "b7f5aed55829f376e4f7e5ea5b80ccdcb023e983",
                            "status": "affected",
                            "versionType": "git"
                        },
                        {
                            "version": "47d902b90a32a42a3d33aef3a02170fc6f70aa23",
                            "lessThan": "e803040b368d046434fbc8a91945c690332c4fcf",
                            "status": "affected",
                            "versionType": "git"
                        },
                        {
                            "version": "47d902b90a32a42a3d33aef3a02170fc6f70aa23",
                            "lessThan": "ba6a9970ce9e284cbc04099361c58731e308596a",
                            "status": "affected",
                            "versionType": "git"
                        },
                        {
                            "version": "47d902b90a32a42a3d33aef3a02170fc6f70aa23",
                            "lessThan": "31edf4bbe0ba27fd03ac7d87eb2ee3d2a231af6d",
                            "status": "affected",
                            "versionType": "git"
                        }
                    ]
                }

{
                    "product": "Linux",
                    "vendor": "Linux",
                    "defaultStatus": "affected",
                    "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
                    "programFiles": [
                        "drivers/block/nbd.c"
                    ],
                    "versions": [
                        {
                            "version": "4.12",
                            "status": "affected"
                        },
                        {
                            "version": "0",
                            "lessThan": "4.12",
                            "status": "unaffected",
                            "versionType": "semver"
                        },
                        {
                            "version": "5.4.273",
                            "lessThanOrEqual": "5.4.*",
                            "status": "unaffected",
                            "versionType": "semver"
                        },
                        {
                            "version": "5.10.214",
                            "lessThanOrEqual": "5.10.*",
                            "status": "unaffected",
                            "versionType": "semver"
                        },
                        {
                            "version": "5.15.153",
                            "lessThanOrEqual": "5.15.*",
                            "status": "unaffected",
                            "versionType": "semver"
                        },
                        {
                            "version": "6.1.83",
                            "lessThanOrEqual": "6.1.*",
                            "status": "unaffected",
                            "versionType": "semver"
                        },
                        {
                            "version": "6.6.23",
                            "lessThanOrEqual": "6.6.*",
                            "status": "unaffected",
                            "versionType": "semver"
                        },
                        {
                            "version": "6.7.11",
                            "lessThanOrEqual": "6.7.*",
                            "status": "unaffected",
                            "versionType": "semver"
                        },
                        {
                            "version": "6.8.2",
                            "lessThanOrEqual": "6.8.*",
                            "status": "unaffected",
                            "versionType": "semver"
                        },
                        {
                            "version": "6.9",
                            "lessThanOrEqual": "*",
                            "status": "unaffected",
                            "versionType": "original_commit_for_fix"
                        }
                    ]
                }