CVE-2019-20056

stb_image.h (aka the stb image loader) 2.23, as used in libsixel and other products, has an assertion failure in stbi__shiftsigned.

CVE-2019-19777

stb_image.h (aka the stb image loader) 2.23, as used in libsixel and other products, has a heap-based buffer over-read in stbi__load_main.