Fix crash when there are no exception listeners

[hach-que]

I ran into a crash when the process had no exception listeners registered. Roughly I was doing something like this:

before(function(callback) { /* ... */ callback(); });
after(function(callback) { /* ... */ callback(); });

parallel(function() {
  // parallel tests as normal
});

[danielstjules]

Thanks! Added in https://github.com/danielstjules/mocha.parallel/commit/963f0b8ecb81494524fcf9a5f170412516bd2559