correct cycle detection in ground/1 (#2073)

src/machine/machine_state_impl.rs

@@ -1648,7 +1648,7 @@ impl MachineState {
                 }
             }
 
-            if value.is_compound(iter.heap) {
+            if value.is_ref() {
                 if visited.contains(&value) {
                     for _ in stack_len..iter.stack_len() {
                         iter.pop_stack();