DEBIAN-CVE-2026-117556: Medium severity debian/zchunk vulnerability

zchunk before 1.5.5 contains a heap out-of-bounds read vulnerability in readpreface() in src/lib/header.c that passes the total header length instead of remaining bytes to compint parsing functions. Attackers can supply a crafted .zck file whose header ends inside a compressed integer, causing reads past the heap header buffer and crashing the application.