🐛 Bug: Mocha throws error if it is running a spec more than once #2706

> mocha.run();

everything ok!

> mocha.run()

TypeError: Cannot read property 'call' of undefined
    at r (https://cdnjs.cloudflare.com/ajax/libs/mocha/3.2.0/mocha.min.js:2:8563)
    at r.run (https://cdnjs.cloudflare.com/ajax/libs/mocha/3.2.0/mocha.min.js:2:9635)
    at n (https://cdnjs.cloudflare.com/ajax/libs/mocha/3.2.0/mocha.min.js:2:13371)
    at https://cdnjs.cloudflare.com/ajax/libs/mocha/3.2.0/mocha.min.js:2:13710
    at i (https://cdnjs.cloudflare.com/ajax/libs/mocha/3.2.0/mocha.min.js:1:572)