feat(c-api) Transmute boxed vecs to Vec<Option<Box<T>>> when deleting.

It's a safer way to handle partially uninitialized boxed vector, since
it protects against based deletion for every item.

lib/c-api/src/wasm_c_api/macros.rs

@@ -345,13 +345,7 @@ See the [`wasm_" $name "_vec_t`] type to get an example."]
                 if let Some(vec) = ptr {
                     if !vec.data.is_null() {
                         let data: Vec<*mut [<wasm_ $name _t>]> = Vec::from_raw_parts(vec.data, vec.size, vec.size);
-
-                        // If the vector has been initialized (we check
-                        // only the first item), we can transmute items to
-                        // `Box`es.
-                        if vec.size > 0 && !data[0].is_null() {
-                            let _data: Vec<Box<[<wasm_ $name _t>]>> = ::std::mem::transmute(data);
-                        }
+                        let _data: Vec<Option<Box<[<wasm_ $name _t>]>>> = ::std::mem::transmute(data);
 
                         vec.data = ::std::ptr::null_mut();
                         vec.size = 0;