DEBIAN-CVE-2026-120830: High severity debian/libslirp vulnerability

A flaw was found in libslirp in icmpforwarderror(). When building an ICMP error response, the length of the quoted original IPv4 packet was taken from the IP header without clamping it to the actual buffer length. A truncated or malformed packet could cause an out-of-bounds heap read, leaking host memory into the ICMP error packet returned to a guest or container.