fixed negative length for empty packages
fixed negative length for empty packages

chkr1011/MQTTnet,JTrotta/MQTTnet,chkr1011/MQTTnet,JTrotta/MQTTnet,JTrotta/MQTTnet,chkr1011/MQTTnet,chkr1011/MQTTnet,chkr1011/MQTTnet,JTrotta/MQTTnet