CVE-2026-44636: libsixel: integer overflow in encoder

libsixel is a SIXEL encoder/decoder implementation derived from kmiya's sixel. From to 1.8.7-r1, signed integer overflow in sixelencodehighcolor's allocation size calculation can lead to a heap buffer overflow. The public sixelencode entry point validates only that width and height are greater than zero, with no upper bound. width and height are multiplied as plain int when computing the allocation size for palettedpixels and normalizedpixels. Any caller that asks libsixel to encode a pixel buffer with width times height greater than INTMAX (about 2.15 billion) will hit a wrapped allocation size; under the right wrap, the malloc succeeds with a buffer much smaller than the encoder expects, and the encoder writes past the end of the heap allocation. This vulnerability is fixed in 1.8.7-r2.