Bitfinex: avoid crash when getOrder is called and no order is found.

[Alexandre-io]

Bitfinex: avoid crash when getOrder is called and no order is found:

exchange.js:492
      if (order.status === 'rejected' && order.reject_reason === 'post only') {
                ^

TypeError: Cannot read property 'status' of undefined
    at Object.getOrder (zenbot/extensions/exchanges/bitfinex/exchange.js:492:17)
    at Timeout.checkHold [as _onTimeout] (zenbot/lib/engine.js:279:28)
    at ontimeout (timers.js:475:11)
    at tryOnTimeout (timers.js:310:5)
    at Timer.listOnTimeout (timers.js:270:5)