Fix infinite loop

Infinite loop happens in case expression with variable operation is
interpreted. In such case it is returned without modification from query
and starts being interpreted again. For example in (cdr-atom ($A $B $C))

lib/src/metta/interpreter2.rs

@@ -499,7 +499,7 @@ fn query<'a, T: SpaceRef<'a>>(space: T, prev: Option<Rc<RefCell<Stack>>>, atom:
         // TODO: This is a hotfix. Better way of doing this is adding
         // a function which modifies minimal MeTTa interpreter code
         // in order to skip such evaluations in metta-call function.
-        return finished_result(atom, bindings, prev);
+        return finished_result(return_not_reducible(), bindings, prev)
     }
     let var_x = &VariableAtom::new("X").make_unique();
     let query = Atom::expr([EQUAL_SYMBOL, atom.clone(), Atom::Variable(var_x.clone())]);
@@ -916,7 +916,7 @@ mod tests {
         #[cfg(feature = "variable_operation")]
         assert_eq!(result, vec![metta_atom("A")]);
         #[cfg(not(feature = "variable_operation"))]
-        assert_eq!(result, vec![expr!(a "A" b)]);
+        assert_eq!(result, vec![NOT_REDUCIBLE_SYMBOL]);
     }