sunrpc: fix null pointer dereference on zero-length checksum

{
                    "product": "Linux",
                    "vendor": "Linux",
                    "defaultStatus": "unaffected",
                    "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
                    "programFiles": [
                        "net/sunrpc/auth_gss/svcauth_gss.c"
                    ],
                    "versions": [
                        {
                            "version": "0653028e8f1c97fec30710813a001ad8a2ec34f4",
                            "lessThan": "81cec07d303186d0d8c623ef8b5ecd3b81e94cf6",
                            "status": "affected",
                            "versionType": "git"
                        },
                        {
                            "version": "0653028e8f1c97fec30710813a001ad8a2ec34f4",
                            "lessThan": "affc03d44921f493deaae1d33151e3067a6f9f8f",
                            "status": "affected",
                            "versionType": "git"
                        },
                        {
                            "version": "0653028e8f1c97fec30710813a001ad8a2ec34f4",
                            "lessThan": "ab9a70cd2386a0d70c164b0905dd66bc9af52e77",
                            "status": "affected",
                            "versionType": "git"
                        },
                        {
                            "version": "0653028e8f1c97fec30710813a001ad8a2ec34f4",
                            "lessThan": "6df164e29bd4e6505c5a2e0e5f1e1f6957a16a42",
                            "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/sunrpc/auth_gss/svcauth_gss.c"
                    ],
                    "versions": [
                        {
                            "version": "6.3",
                            "status": "affected"
                        },
                        {
                            "version": "0",
                            "lessThan": "6.3",
                            "status": "unaffected",
                            "versionType": "semver"
                        },
                        {
                            "version": "6.6.112",
                            "lessThanOrEqual": "6.6.*",
                            "status": "unaffected",
                            "versionType": "semver"
                        },
                        {
                            "version": "6.12.53",
                            "lessThanOrEqual": "6.12.*",
                            "status": "unaffected",
                            "versionType": "semver"
                        },
                        {
                            "version": "6.17.3",
                            "lessThanOrEqual": "6.17.*",
                            "status": "unaffected",
                            "versionType": "semver"
                        },
                        {
                            "version": "6.18",
                            "lessThanOrEqual": "*",
                            "status": "unaffected",
                            "versionType": "original_commit_for_fix"
                        }
                    ]
                }