/usr/share/doc/libmad0-dev/CREDITS.gz is in libmad0-dev 0.15.1b-8ubuntu1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/doc/libmad0-dev/../libmad0/CREDITS.gz
/usr/share/doc/libmad0-dev/../libmad0/CREDITS.gz