Imagemagick Has Global Buffer Overflow (OOB Read) via Negative Pixel Index in UIL and XPM Writer

ImageMagick is free and open-source software used for editing and manipulating digital images. Prior to versions 7.1.2-15 and 6.9.13-40, the UIL and XPM image encoder do not validate the pixel index value returned by `GetPixelIndex()` before using it as an array subscript. In HDRI builds, `Quantum` is a floating-point type, so pixel index values can be negative. An attacker can craft an image with negative pixel index values to trigger a global buffer overflow read during conversion, leading to information disclosure or a process crash. Versions 7.1.2-15 and 6.9.13-40 contain a patch.