Double-free in stbi__load_gif_main_outofmem in stb_image

stb_image is a single file MIT licensed library for processing images. A crafted image file can trigger `stbi__load_gif_main_outofmem` attempt to double-free the out variable. This happens in `stbi__load_gif_main` because when the `layers * stride` value is zero the behavior is implementation defined, but common that realloc frees the old memory and returns null pointer. Since it attempts to double-free the memory a few lines below the first “free”, the issue can be potentially exploited only in a multi-threaded environment. In the worst case this may lead to code execution.