cluster: exception in master doesn't kill workers

[bnoordhuis]

To reproduce:

var assert = require('assert');
var cluster = require('cluster');

if (cluster.isMaster) {
  cluster.fork();
  throw new Error('kill master'); // should kill the worker too
} else {
  setTimeout(function() {
    assert(false, 'worker should have been killed');
  }, 2500);
}

The workers array is empty in the uncaughtException listener that startMaster() installs so the listener doesn't actually kill anything.