Bound pointer is always null when method is invoked

Please include the following in your bug report:

**Version of emscripten/emsdk:**
emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 3.1.39 (36f871819b566281d160470a1ec4515f379e9f77)
clang version 17.0.0 (https://github.com/llvm/llvm-project c672c3fe05adbb590abc99da39143b55ad510538)
Target: wasm32-unknown-emscripten
Thread model: posix

**Repro**

Address and Undefined Sanitizer both issue diagnostics about this.

```
cat > index.html <

Testcase

Module().then(instance => {
MyModule = instance;

theScopedValueFactory = new MyModule.ScopedValueFactory();
let vec = new theScopedValueFactory.makeVector(1, 1, 1);
console.log(vec.x(), vec.y());
});

EOF

cat > myBindings.idl < myBindings.cpp <