/usr/share/doc/libavcodec57/changelog.Debian.gz is in libavcodec57 7:3.4.2-2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/doc/libavcodec57/../libavutil55/changelog.Debian.gz
/usr/share/doc/libavcodec57/../libavutil55/changelog.Debian.gz