correct for issues #88, #89, #90, conformity test #208.

src/prolog/machine/machine_state_impl.rs

@@ -1905,6 +1905,7 @@ impl MachineState {
 
                 match d {
                     Addr::Con(Constant::Atom(..)) | Addr::Con(Constant::Char(_)) => self.p += 1,
+                    Addr::Con(Constant::EmptyList) => self.p += 1,
                     _ => self.fail = true
                 };
             },

src/prolog/machine/system_calls.rs

@@ -362,7 +362,7 @@ impl MachineState {
                     },
                     ref addr if addr.is_ref() => {
                         let stub = MachineError::functor_stub(clause_name!("atom_chars"), 2);
-
+                        
                         match self.try_from_list(temp_v!(2), stub.clone()) {
                             Err(e) => return Err(e),
                             Ok(addrs) => {