Fix the rest of pdu tests

pdu/pdu.go

@@ -3,6 +3,7 @@ package pdu
 import (
 	"bytes"
 	"encoding/binary"
+	"fmt"
 )
 
 type (
@@ -64,6 +65,9 @@ func (p *PDU_HEADER) Encode(buf *bytes.Buffer) error {
 	return nil
 }
 func (p *PDU_HEADER) Decode(buf *bytes.Buffer) error {
+	if buf == nil {
+		return fmt.Errorf("cannot decode nil buffer")
+	}
 	binary.Read(buf, binary.BigEndian, &p.command_length)
 	binary.Read(buf, binary.BigEndian, &p.command_id)
 	binary.Read(buf, binary.BigEndian, &p.command_status)