repl: fixing `undefined` in invalid REPL keyword error

When an invalid REPL keyword is used, we actually print `undefined` as
well in the console.

    > process.version
    'v2.3.4'
    > .invalid_repl_command
    Invalid REPL keyword
    undefined
    >

This patch prevents printing `undefined` in this case.

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

PR-URL: https://github.com/nodejs/io.js/pull/2163
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>

lib/repl.js

@@ -342,7 +342,12 @@ function REPLServer(prompt,
       self.bufferedCommand = '';
 
       // If we got any output - print it (if no error)
-      if (!e && (!self.ignoreUndefined || ret !== undefined)) {
+      if (!e &&
+          // When an invalid REPL command is used, error message is printed
+          // immediately. We don't have to print anything else. So, only when
+          // the second argument to this function is there, print it.
+          arguments.length === 2 &&
+          (!self.ignoreUndefined || ret !== undefined)) {
         self.context._ = ret;
         self.outputStream.write(self.writer(ret) + '\n');
       }

test/parallel/test-repl.js

@@ -189,7 +189,11 @@ function error_test() {
     { client: client_unix, send: 'url.format("http://google.com")',
       expect: 'http://google.com/' },
     { client: client_unix, send: 'var path = 42; path',
-      expect: '42' }
+      expect: '42' },
+    // this makes sure that we don't print `undefined` when we actually print
+    // the error message
+    { client: client_unix, send: '.invalid_repl_command',
+      expect: 'Invalid REPL keyword\n' + prompt_unix },
   ]);
 }