MarshalToWriter does not pool buffers since Go 1.17

[martin-sucha]

Go 1.17 changed the behavior of net.Buffers in golang/go#45163. This causes code at

easyjson/buffer/pool.go

Lines 171 to 180 in 11c9d7f

	bufs := net.Buffers(b.bufs)
	if len(b.Buf) > 0 {
	bufs = append(bufs, b.Buf)
	}
	n, err := bufs.WriteTo(w)
	for _, buf := range b.bufs {
	putBuf(buf)
	}
	putBuf(b.toPool)

to not put buffers back to the pool.
Argument to putBuf is nil because WriteTo sets all elements of the b.bufs slice to nil.