unix: don't clear flags after closing UDP handle

src/unix/udp.c

@@ -79,7 +79,6 @@ void uv__udp_finish_close(uv_udp_t* handle) {
   }
 
   /* Now tear down the handle. */
-  handle->flags = 0;
   handle->recv_cb = NULL;
   handle->alloc_cb = NULL;
   /* but _do not_ touch close_cb */

test/test-udp-send-and-recv.c

@@ -53,6 +53,7 @@ static uv_buf_t alloc_cb(uv_handle_t* handle, size_t suggested_size) {
 
 static void close_cb(uv_handle_t* handle) {
   CHECK_HANDLE(handle);
+  ASSERT(uv_is_closing(handle));
   close_cb_called++;
 }