Readline should emit the keypress event if its not null.

SIGINT event is handled before the CTRL+C's keypress. Inquirer breaks force closes by trying to emit an event to a null readline object. Reproducible on `yeoman`, stack trace is below:
~~~
TypeError: Cannot call method 'emit' of null
    at Object.<anonymous> (/Users/burcu/yo/node_modules/yeoman-generator/node_modules/inquirer/lib/inquirer.js:119:11)
    at ReadStream.EventEmitter.emit (events.js:117:20)
    at emitKey (readline.js:1089:12)
    at ReadStream.onData (readline.js:834:14)
    at ReadStream.EventEmitter.emit (events.js:95:17)
    at ReadStream.<anonymous> (_stream_readable.js:720:14)
    at ReadStream.EventEmitter.emit (events.js:92:17)
    at emitReadable_ (_stream_readable.js:392:10)
    at emitReadable (_stream_readable.js:388:5)
    at readableAddChunk (_stream_readable.js:150:9)
~~~