remove unstandard error

All unmarshall calls return the same errShortPacket except this one for
no reason. Fix that.
diff --git a/packet.go b/packet.go
index e2dec7a..c5c32fc 100644
--- a/packet.go
+++ b/packet.go
@@ -802,7 +802,7 @@
 	} else if p.Length, b, err = unmarshalUint32Safe(b); err != nil {
 		return err
 	} else if uint32(len(b)) < p.Length {
-		return errors.New("truncated packet")
+		return errShortPacket
 	}
 
 	p.Data = make([]byte, p.Length)