Crash in module_wrap.cc with malformed loader

loader.mjs:

```js
export function transformSource() {
  return {
    source: {
      boom: true,
    },
  };
}
```

test.mjs:

```js
import fs from 'fs';
```

```
> node --loader ./loader.mjs test.mjs
C:\Program Files\PowerShell\7\pwsh.exe[28872]: c:\ws\src\module_wrap.cc:131: Assertion `args[2]->IsString()' failed.
```