wasm: When having multiple theads that share a mutex, then it is possible that a mutex will stay locked despite having had unlock() called on it

When having multiple theads that share a mutex, then it is possible that a mutex will stay locked despite having had unlock() called on it. This happens in the attached example when the mutex is unlocked after postEvent() is called. If the mutex is unlocked before postEvent() is called then it works fine. This also works fine on the desktop.