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.