repl: fix crashes when buffering command

Wrong order of operands was causing problems while trying to use command
buffering:

    > {
    ...   a: 3,
    ...

    repl.js:284
            if (cmd.trim().match(/^npm /) && !self.bufferedCommand) {
                    ^
    TypeError: Cannot call method 'trim' of undefined
        at finish (repl.js:284:17)
        at REPLServer.self.eval (repl.js:118:5)
        at rli.on.e (repl.js:260:20)
        at REPLServer.self.eval (repl.js:118:5)
        at Interface.<anonymous> (repl.js:250:12)
        at Interface.EventEmitter.emit (events.js:88:17)
        at Interface._onLine (readline.js:183:10)
        at Interface._line (readline.js:502:8)
        at Interface._ttyWrite (readline.js:720:14)
        at ReadStream.<anonymous> (readline.js:105:12)

Test included.

Closes #3515.
Closes #3517.
Closes #3621.