events: avoid emit() eager deopt

This commit makes sure EventEmitter.emit() doesn't get deoptimized by
V8. The deopt happens when accessing out of bound indexes of the
`arguments` object.

This issue has been raised here: nodejs#10323 and this specific case might
become a more serious performance issue in upcoming V8 releases.

lib/events.js

@@ -148,7 +148,8 @@ EventEmitter.prototype.emit = function emit(type) {
 
  // If there is no 'error' event listener then throw.
  if (doError) {
- er = arguments[1];
+ if (arguments.length > 1)
+ er = arguments[1];
  if (domain) {
  if (!er)
  er = new Error('Uncaught, unspecified "error" event');