CVE-2026-33019: libsixel: Integer overflow leads to Out-of-bounds Read in img2sixel

libsixel is a SIXEL encoder/decoder implementation derived from kmiya's sixel. Versions 1.8.7 and prior contain an integer overflow leading to an out-of-bounds heap read in the --crop option handling of img2sixel, where positive coordinates up to INTMAX are accepted without overflow-safe bounds checking. In sixelencoderdoclip(), the expression clipw + clipx overflows to a large negative value when clipx is INTMAX, causing the bounds guard to be skipped entirely, and the unclamped coordinate is passed through sixelframeclip() to clip(), which computes a source pointer far beyond the image buffer and passes it to memmove(). An attacker supplying a specially crafted crop argument with any valid image can trigger an out-of-bounds read in the heap, resulting in a reliable crash and potential information disclosure. This issue has been fixed in version 1.8.7-r1.