avoid a potential type error

lib/write-queue.js

@@ -17,7 +17,7 @@ module.exports = function () {
 
     var next = function (id) {
         setTimeout(function () {
-            if (map[id] && map[id].length === 0) { return void delete map[id]; }
+            if (!map[id] || map[id].length === 0) { return void delete map[id]; }
             var task = map[id].shift();
             task(fix1(next, id));
         });