/usr/lib/x86_64-linux-gnu/liblavfile.so is in libmjpegtools-dev 1:2.1.0+debian-5.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/liblavfile-2.1.so.0.0.0
/usr/lib/x86_64-linux-gnu/liblavfile-2.1.so.0.0.0