pptp: ensure minimal skb length in pptp_xmit()

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                    "vendor": "Linux",
                    "defaultStatus": "unaffected",
                    "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
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                        "drivers/net/ppp/pptp.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/ppp/pptp.c"
                    ],
                    "versions": [
                        {
                            "version": "2.6.12",
                            "status": "affected"
                        },
                        {
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                            "lessThanOrEqual": "5.4.*",
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                        },
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                            "version": "6.1.148",
                            "lessThanOrEqual": "6.1.*",
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                        {
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                        },
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                            "lessThanOrEqual": "6.16.*",
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                        {
                            "version": "6.17",
                            "lessThanOrEqual": "*",
                            "status": "unaffected",
                            "versionType": "original_commit_for_fix"
                        }
                    ]
                }