CVE-2019-12217: affects libsdl-image, not libsdl

Very similar to CVE-2019-12221. The vulnerability lies in the sdl_image
code base.

As for other libsdl-image vulnerabilities, the paths are very different
so it is very unlikely that MITRE will accept to reject them as
duplicates.

See patch proposal and report upstream side:
https://bugzilla.libsdl.org/show_bug.cgi?id=4626#c1