I noticed that in OopHandle/WeakHandle we have {replace,xchg,cmpxchg} methods that overwrite the handle. This is only safe to do when the handle is not empty -- i.e. when there is a storage allocated for it in relevant OopStorage. Otherwise we attempt the store to nullptr, and get a SEGV.

Only OopHandle::replace does the assertion for this. We need to add these asserts everywhere else.