Add CVE-2019-5721/wireshark

Issue was adressed in 2.5.1 upstream with the upstream commit

https://code.wireshark.org/review/gitweb?p=wireshark.git;a=commit;h=177962a5b4a05759b40fb6fc07a4a6eec306a9bf

and cherry-picked for the 2.4.x branch.

2.6.1-1 was the first version in the archive via unstable containing the
change.

For stretch later on wireshark got rebased on rebuild from the unstable
version. The first one in the 2.6.x series on this strategy was
2.6.3-1~deb9u1.

Mark first version via stretch/stretch-security beeing 2.6.3-1~deb9u1 as
the fixed version.