wasmtime-c-api: Don't create slices with null pointers (#1492)

It's a common idiom to pass in `NULL` for slices of zero-length in the C
API, but it's not safe to create a Rust `&[T]` slice with this `NULL`
pointer. Special-case this in the `as_slice()` method of incoming
vectors to return an empty slice so we don't violate Rust's invariants.

crates/c-api/src/vec.rs

@@ -42,8 +42,16 @@ macro_rules! declare_vecs {
             }
 
             pub fn as_slice(&self) -> &[$elem_ty] {
+                // Note that we're careful to not create a slice with a null
+                // pointer as the data pointer, since that isn't defined
+                // behavior in Rust.
+                if self.size == 0 {
+                    &[]
+                } else {
+                    assert!(!self.data.is_null());
                     unsafe { slice::from_raw_parts(self.data, self.size) }
                 }
+            }
 
             pub fn take(&mut self) -> Vec<$elem_ty> {
                 if self.data.is_null() {