There's a bug in the implementation of Wasm GC struct instructions such as `struct.get` and `struct.set`, in that they don't correctly implement null ref checks.

Here's an example test case (can run in the JSTests/wasm/gc test directory):

```
import * as assert from "../assert.js";
import { compile, instantiate } from "./wast-wrapper.js";

let m = instantiate(`
  (module
    (type (struct (field i32)))
    (func (export "f") (result i32)
      (struct.get 0 0 (ref.null 0)))
  )
`);

m.exports.f();
```

This currently crashes, instead of raising a trap exception as needed.