Don't write to Async objects after we've deleted them

The handle here is actually the uv_async_t object named watcher that
is part of the Async object so we shouldn't try and write to it after
the Async object has been freed.

src/async.h

@@ -50,7 +50,6 @@ public:
         assert(handle->data != NULL);
         Async* async = static_cast<Async*>(handle->data);
         delete async;
-        handle->data = NULL;
     }
 
     void finish() {

src/statement.cc

@@ -647,7 +647,6 @@ void Statement::CloseCallback(uv_handle_t* handle) {
     assert(handle->data != NULL);
     Async* async = static_cast<Async*>(handle->data);
     delete async;
-    handle->data = NULL;
 }
 
 void Statement::AsyncEach(uv_async_t* handle, int status) {