/usr/lib/x86_64-linux-gnu/libavcodec.so.57 is in libavcodec57 7:3.2.12-1~deb9u1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libavcodec.so.57.64.101
/usr/lib/x86_64-linux-gnu/libavcodec.so.57.64.101