Disordered output from FIFO queue

Code here: [https://go.dev/play/p/FZMhfuOolps](https://go.dev/play/p/FZMhfuOolps)
I ran it 10 times and 4 times reported disordered.
Example abnormal output:
```
panic: disordered result: 449, expected: 448

goroutine 1 [running]:
main.main()
	/tmp/sandbox1192096132/prog.go:58 +0x3d6

Program exited.
```