libav - security update

Several security issues have been corrected in multiple demuxers and
decoders of the libav multimedia library. A full list of the changes is
available at <https://git.libav.org/?p=libav.git;a=blob;f=Changelog;hb=refs/tags/v11.4&gt;

For the stable distribution (jessie), these problems have been fixed in
version 6:11.4-1~deb8u1.

For the testing distribution (stretch), these problems have been fixed
in version 6:11.4-1.

For the unstable distribution (sid), these problems have been fixed in
version 6:11.4-1.

We recommend that you upgrade your libav packages.