func NewUNSUBACKFromBytes(fixedHeader FixedHeader, variableHeader []byte) (Packet, error) {
	if err := validateUNSUBACKBytes(fixedHeader, variableHeader); err != nil {
		return nil, err
	}
	packetID, _ := decodeUint16(variableHeader)
	p := &UNSUBACK{
		PacketID: packetID,
	}
	p.fixedHeader = fixedHeader
	p.variableHeader = variableHeader
	return p, nil
}
NewUNSUBACKFromBytes creates an UNSUBACK Packet
from the byte data and returns it.
package packet
const lenUNSUBACKFixedHeader = 2
const lenUNSUBACKVariableHeader = 2
type UNSUBACK struct {
	base
	PacketID uint16
}