stb_image.h (stbi__start_mem): initialize io.skip and io.eof to NULL

Patch authored by @NBickford-NV: https://github.com/miniupnp/stb/pull/1
c.f.: https://github.com/nothings/stb/pull/1751

(cherry picked from commit ab900a389f9888300f67250e275d5b3198de7f09)

src/video/stb_image.h

@@ -895,6 +895,8 @@ static void stbi__refill_buffer(stbi__context *s);
 static void stbi__start_mem(stbi__context *s, stbi_uc const *buffer, int len)
 {
    s->io.read = NULL;
+   s->io.skip = NULL;
+   s->io.eof = NULL;
    s->read_from_callbacks = 0;
    s->callback_already_read = 0;
    s->img_buffer = s->img_buffer_original = (stbi_uc *) buffer;