Null pointer dereference in test case webp_data::tests::test_encoder

[icmccorm]

I've been developing an experimental version of Miri that can execute foreign functions by interpreting LLVM bytecode.

Miri found the following error when executing the test webp_data::tests::test_encoder at version 0.9.0 of webp-animation.

error: Undefined Behavior: dereferencing pointer failed: null pointer is a dangling pointer (it has no provenance)
   --> .../rust/library/core/src/slice/raw.rs:101:9
    |
101 |         &*ptr::slice_from_raw_parts(data, len)
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ dereferencing pointer failed: null pointer is a dangling pointer (it has no provenance)
    |
    = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
    = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
    = note: BACKTRACE:
    = note: inside `std::slice::from_raw_parts::<'_, u8>` at .../rust/library/core/src/slice/raw.rs:101:9: 101:47
note: inside `webp_data::WebPData::as_slice`
   --> src/webp_data.rs:27:18
    |
27  |         unsafe { slice::from_raw_parts(self.data.bytes, self.data.size) }
    |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: inside `<webp_data::WebPData as std::ops::Deref>::deref`
   --> src/webp_data.rs:41:9
    |
41  |         self.as_slice()
    |         ^^^^^^^^^^^^^^^
note: inside `webp_data::tests::test_encoder`
   --> src/webp_data.rs:58:20
    |
58  |         assert_eq!(data.len(), 0);
    |                    ^^^^^^^^^^
note: inside closure
   --> src/webp_data.rs:56:23
    |
55  |     #[test]
    |     ------- in this procedural macro expansion
56  |     fn test_encoder() {
    | 

It seems like WebPData::new() does not actually initialize the WebPData struct to point to valid memory. In particular, here's where data is initialized in Rust:

let mut data = mem::zeroed();
webp::WebPDataInit(&mut data);
data

It seems like webp::WebPDataInit just zero-initializes the memory that it's passed:

static inline void WebPDataInit(WebPData* webp_data) {
  if (webp_data != ((void*)0)) {
    memset(webp_data, 0, sizeof(*webp_data));
  }
}

However, this shouldn't be necessary, since the memory being referenced has already been initialized by mem::zeroed() in Rust. Anyway, it seems like the implementation of WebPData::as_slice needs to account for the case when data is NULL.