test: handle zero-length udp datagram

Under rare but benign circumstances, incoming datagrams are dropped by
the operating system after libuv has been notified of their arrival but
before libuv has had a chance to receive them.

Fixes: libuv#4219

test/test-udp-recv-in-a-row.c

@@ -50,11 +50,15 @@ static void sv_recv_cb(uv_udp_t* handle,
                        const uv_buf_t* rcvbuf,
                        const struct sockaddr* addr,
                        unsigned flags) {
-  if (++ recv_cnt < N) {
-    ASSERT_EQ(sizeof(send_data), nread);
-  } else {
-    ASSERT_OK(nread);
-  }
+  /* |nread| can be zero when the kernel drops an incoming datagram after
+   * marking the file descriptor as readable but before libuv has a chance
+   * to receive it. Libuv still invokes the uv_udp_recv_cb callback to give
+   * back the memory from the uv_alloc_cb callback.
+   *
+   * See https://github.com/libuv/libuv/issues/4219.
+   */
+  ASSERT_GE(nread, 0);
+  recv_cnt++;
 }
 
 static void check_cb(uv_check_t* handle) {