bug: enemie movement sometimes deref nullptr when calling get_pos()

fixed level gen

src/enemies.cc

@@ -79,6 +79,10 @@ void new_enemy(RNG *rng, std::unique_ptr<enemy_base> &p,
                 p = make_unique<merchant>(rng, enabled_features, pos, which_room);
                 break;
 
+        case rhalfling:
+                p = make_unique<halfling>(rng, enabled_features, pos, which_room);
+                break;
+
         default:
                 break;
         }