6696264: assert("narrow oop can never be zero") for GCBasher & ParNewGC

Decouple set_klass() with zeroing the gap when compressed.

Reviewed-by: kvn, ysr, jrose

hotspot/src/share/vm/memory/space.cpp

@@ -815,6 +815,7 @@ void ContiguousSpace::allocate_temporary_filler(int factor) {
            "size for smallest fake object doesn't match");
     instanceOop obj = (instanceOop) allocate(size);
     obj->set_mark(markOopDesc::prototype());
+    obj->set_klass_gap(0);
     obj->set_klass(SystemDictionary::object_klass());
   }
 }