hi,I see the  code for method setBytes in class com.alibaba.dubbo.remoting.transport.netty4.NettyBackedChannelBuffer.java which is below,is there a problem ?

![image](https://user-images.githubusercontent.com/20593632/46648646-90eaba00-cbc8-11e8-896d-9a1d235d7e0b.png)


private ByteBuf buffer;
    public NettyBackedChannelBuffer(ByteBuf buffer) {
        Assert.notNull(buffer, "buffer == null");
        this.buffer = buffer;
    }
    
    @Override
    public int capacity() {
        return buffer.capacity();
    }
    
    @Override
    public ChannelBuffer copy(int index, int length) {
        return new NettyBackedChannelBuffer(buffer.copy(index, length));
    }
    
    @Override
    public ChannelBufferFactory factory() {
        return null;
    }
    
    @Override