reuse buffers instead of allocating new ones during each read

[hessonsu]

issues 337

[hessonsu]

// Unmarshal decodes an interleaved frame.
func (f *InterleavedFrame) Unmarshal(br *bufio.Reader) error {
	var header [4]byte
	_, err := io.ReadFull(br, header[:])
	if err != nil {
		return err
	}

	if header[0] != InterleavedFrameMagicByte {
		return fmt.Errorf("invalid magic byte (0x%.2x)", header[0])
	}

	// it's useless to check payloadLen since it's limited to 65535
	payloadLen := int(uint16(header[2])<<8 | uint16(header[3]))

	f.Channel = int(header[1])
	f.Payload = make([]byte, payloadLen)

	_, err = io.ReadFull(br, f.Payload)
	return err
}

Discuss, how to use this method not to request memory make([]byte, payloadLen)
This increases the pressure on the GC

[aler9]

this is not a memory leak but a possible improvement, that is the ability to reuse buffers instead of allocating new ones.