Null pointer dereference because of an uninitialized variable in stb_image

stb_image is a single file MIT licensed library for processing images. If `stbi__load_gif_main` in `stbi_load_gif_from_memory` fails it returns a null pointer and may keep the `z` variable uninitialized. In case the caller also sets the flip vertically flag, it continues and calls `stbi__vertical_flip_slices` with the null pointer result value and the uninitialized `z` value. This may result in a program crash.