Fix endless recursion in Class_obj::GetInstanceFields

[profelis]

cpp::Pointer.mSuper points to same cpp::Pointer instance

Steps to reproduce:

1. get this repo and build debug sample https://github.com/foreignsasquatch/raylib-hx/tree/main/examples
2. add breakpoint inside while loop https://github.com/foreignsasquatch/raylib-hx/blob/main/examples/BasicWindow.hx#L12
3. start debugger session in vscode

stacktrace
https://gist.github.com/profelis/390f80bc47afc16f41b3afced4896cc6

[Simn]

I don't really understand how a pointer relation like this happens in the first place, but this kind of check is usually a good idea anyway, so I'm happy to merge it.