Add fixed version for CVE-2015-1208/ffmpeg

The first version uploaded to unstable, 7:2.5-1, after the 2.4.x series
did not contain the fix, neither 2.5.1 upstream, only then later in
2.5.3.

I have not verfied that in later series, the fix got not lost again and
possibly then reintroduced via a bugfix release.

Thus please double-check this CVE entry.