Fix REPL crash on tabbing 'this.'

Thanks to Tim Becker for pointing this out.

lib/repl.js

@@ -318,7 +318,8 @@ REPLServer.prototype.complete = function (line) {
           if (typeof obj === "object" || typeof obj === "function") {
             memberGroups.push(Object.getOwnPropertyNames(obj));
           }
-          var p = obj.constructor.prototype; // works for non-objects
+          // works for non-objects
+          var p = obj.constructor ? obj.constructor.prototype : null;
           try {
             var sentinel = 5;
             while (p !== null) {