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

http://git.libav.org/?p=libav.git;a=blob;f=Changelog;hb=refs/tags/v0.8.15

For the stable distribution (wheezy), this problem has been fixed in
version 6:0.8.16-1.

For the testing distribution (jessie), this problem has been fixed in
version 6:11~alpha2-1.

For the unstable distribution (sid), this problem has been fixed in
version 6:11~alpha2-1.

We recommend that you upgrade your libav packages.