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>