Add fixed version via unstable for CVE-2019-5057/sdl-image1.2

Looks we either missed to track the fix or details on the fix got
clarified later. In any case the changeset 7453e79c8cdb references
TALOS-2019-0841 which in turns is associated with CVE-2019-5057 so the
fixed version should be correct for all tracked suites.