pptp: ensure minimal skb length in pptp_xmit()

These records connect through source-stated identifiers. Shared identifiers or dates alone do not establish shared exposure, root cause or applicability.

CVE-2025-38574 · Microsoft / collected KEV context

cve-program · CVE-2025-38574

• https://git.kernel.org/stable/c/97b8c5d322c5c0038cac4bc56fdbe237d0be426f
• https://git.kernel.org/stable/c/26672f1679b143aa34fca0b6046b7fd0c184770d
• https://git.kernel.org/stable/c/5de7513f38f3c19c0610294ee478242bea356f8c
• https://git.kernel.org/stable/c/504cc4ab91073d2ac7404ad146139f86ecee7193
• https://git.kernel.org/stable/c/5005d24377378a20e5c0e53052fc4ebdcdcbc611
• https://git.kernel.org/stable/c/ea99b88b1999ebcb24d5d3a6b7910030f40d3bba
• https://git.kernel.org/stable/c/b7dcda76fd0615c0599c89f36873a6cd48e02dbb
• https://git.kernel.org/stable/c/1a04db0fd75cb6034fc27a56b67b3b8b9022a98c
• https://git.kernel.org/stable/c/de9c4861fb42f0cd72da844c3c34f692d5895b7b

No related operational record is retained in the current change model.