add '', type checking for incomplete lists in sort, keysort (re: #17)

src/prolog/machine/machine_state.rs

@@ -554,6 +554,12 @@ pub(crate) trait CallPolicy: Any {
             },
             &ClauseType::Inlined(ref inlined) => {
                 machine_st.execute_inlined(inlined, &vec![temp_v!(1), temp_v!(2)]);
+                Ok(())
+            },
+            &ClauseType::SkipMaxList => {
+                machine_st.skip_max_list();
+                machine_st.p += 1;
+                
                 Ok(())
             }
         }