libvips: Possible heap-based buffer read overflow when decoding a well-crafted RLE Radiance image

libvips is a fast image processing library with low memory needs. Prior to version 8.18.3, the old-style Radiance RLE decoder in libvips/foreign/radiance.c can process a repeat marker at the beginning of a scanline in scanline_read_old and read q[-1] before any prior pixel exists. A crafted Radiance image loaded through VipsForeignLoadRad can therefore disclose four bytes of adjacent heap data, most likely other image data. This issue is fixed in version 8.18.3.