Fix benchmark parallel in Node 6

Node 6 recently changed callbacks omitted to throw. There are
actually discussions about warning instead now but for now this
fixes the benchmarks.

Fixes petkaantonov#1165

benchmark/lib/dummy.js

@@ -2,7 +2,7 @@
 // with the callback at the Nth position
 exports.dummy = function dummy(n) {
     return function dummy_n() {
-        var cb = arguments[n - 1];
+        var cb = arguments[n - 1] || function(){};
         if (global.asyncTime)
             setTimeout(cb, global.asyncTime || 100);
         else