Instantiating a Ephemeron class with zero instance variables crash the VM

[tesonep]

If a new instance of a Ephemeric class with zero instance variables is created, the GC crashes when marking the objects. The case can be reproduced by the following events:

[
    HFAllocationEvent {
        #oop : 18785504,
        #hash : 17857,
        #id : 22,
        #objectType : #ephemeron,
        #objectSize : 0,
        #isNew : false,
        #isRoot : true
    },
    HFOldSpaceCollectionEvent { }
]