CVE-2019-12219: affects libsdl-image, not libsdl

Very similar to CVE-2019-12220 and CVE-2019-12222. The vulnerability
lies in the sdl_image code base.

Those three CVEs are most likely duplicates, but for some reason the
paths are different. It is very unlikely that MITRE will accept to
reject them as duplicates.

See patch proposal and report on upstream bug tracker:
https://bugzilla.libsdl.org/show_bug.cgi?id=4625#c1