Set watcher.next to null after it's removed from dumpQueue

src/node_io_watcher.cc

@@ -488,6 +488,7 @@ void IOWatcher::Dump(EV_P_ ev_prepare *watcher, int revents) {
 
       // Drop the watcher_obj from the list.
       watcher_obj_last->Set(next_sym, watcher_obj->Get(next_sym));
+      watcher_obj->Set(next_sym, Null());
 
       // Emit drain event
       if (watcher_obj->Has(ondrain_sym)) {

test/simple/test-dumper.js

@@ -59,6 +59,8 @@ function test (N, b, cb) {
       x = x.next;
     }
 
+    assert.equal(null, w.next);
+
     ncomplete++;
     if (cb) cb();
   });