找出池化内存里的 bug

using System;
using System.Buffers;

static ReadOnlyMemory<byte> RentHeader()
{
    byte[] buffer = ArrayPool<byte>.Shared.Rent(3);
    try
    {
        buffer[0] = 72;
        buffer[1] = 68;
        buffer[2] = 82;
        return buffer.AsMemory(0, 3);
    }
    finally
    {
        ArrayPool<byte>.Shared.Return(buffer);
    }
}