CVE-2025-38574 pptp: ensure minimal skb length in pptp_xmit()

In the Linux kernel, the following vulnerability has been resolved: pptp: ensure minimal skb length in pptpxmit Commit aabc6596ffb3 "net: ppp: Add bound checking for skb data on pppsynctxmung" fixed pppsynctxmunge We need a similar fix in pptpxmit, otherwise we might read uninit data as reported ...