repl: preventing REPL crash with inherited properties

When an inherited property is used as a REPL keyword, the REPL crashes.

    ➜  Desktop  iojs
    > process.version
    'v2.3.4'
    > .toString
    readline.js:913
            stream[ESCAPE_DECODER].next(r[i]);
                                    ^
    TypeError: Cannot read property 'call' of undefined
        at REPLServer.parseREPLKeyword (repl.js:746:15)
        at REPLServer.<anonymous> (repl.js:284:16)
        at emitOne (events.js:77:13)
        at REPLServer.emit (events.js:169:7)
        at REPLServer.Interface._onLine (readline.js:210:10)
        at REPLServer.Interface._line (readline.js:549:8)
        at REPLServer.Interface._ttyWrite (readline.js:826:14)
        at ReadStream.onkeypress (readline.js:105:10)
        at emitTwo (events.js:87:13)
        at ReadStream.emit (events.js:172:7)
    ➜  Desktop

This patch makes the internal `commands` object inherit from `null` so
that there will be no inherited properties.

    > process.version
    'v2.3.5-pre'
    > .toString
    Invalid REPL keyword
    >

PR-URL: #2163
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>

lib/repl.js

@@ -217,7 +217,7 @@ function REPLServer(prompt,
 
   self.setPrompt(prompt !== undefined ? prompt : '> ');
 
-  this.commands = {};
+  this.commands = Object.create(null);
   defineDefaultCommands(this);
 
   // figure out which "writer" function to use

test/parallel/test-repl.js

@@ -194,6 +194,10 @@ function error_test() {
     // the error message
     { client: client_unix, send: '.invalid_repl_command',
       expect: 'Invalid REPL keyword\n' + prompt_unix },
+    // this makes sure that we don't crash when we use an inherited property as
+    // a REPL command
+    { client: client_unix, send: '.toString',
+      expect: 'Invalid REPL keyword\n' + prompt_unix },
   ]);
 }