X go/neo/neonet: Fix thinko in recvPktN

We were retruning packet with tail read for next packet data.
Previously decoding was forgiving, but recent rework along msgpack
support added check that is now failing, e.g.

    === RUN   TestEmptyDB/py/!ssl
    I: runneo.py: /tmp/neo214694750/1 !ssl: started master(s): 127.0.0.1:30621
    127.0.0.1:42266 > 127.0.0.1:30621: .1 RequestIdentification &{CLIENT ?(0)0  1 ø [] []}
    127.0.0.1:42266 < 127.0.0.1:30621: (N: decode header: len(payload) != msgLen) 00 00 00 01 80 01 00 00 00 09 00 f0 00 00 01 e0 00 00 01 00 00 00 00 00 06 00 00 00 58 41 d7 ff 69 82 0a 91 25 00 00 00 03 02 00 00 00 00 e0 00 00 01 02 41 d7 ff 69 82 0a 78 ff 01 00 00 00 09 31 32 37 2e 30 2e 30 2e 31 96 f7 00 00 00 01 02 41 d7 ff 69 81 c6 65 f5 00 00 00 00 09 31 32 37 2e 30 2e 30 2e 31 77 9d f0 00 00 01 02 ff ff ff ff ff ff ff ff 00 00 00 02 00 0a 00 00 00 19 00 00 00 00 00 00 00 01 00 00 00 00 00 00 00 01 00 00 00 01 00 00 00 01 01

-> Fix is: pkt.data should be data[:pktLen], not data[:n] since n is how
much we have read at all.

go/neo/neonet/connection.go

@@ -1296,7 +1296,7 @@ func (nl *NodeLink) recvPktN() (*pktBuf, error) {
 	}
 
 	// fixup data/pkt
-	data = data[:n]
+	data = data[:pktLen]
 	pkt.data = data
 
 	return pkt, nil
@@ -1411,7 +1411,7 @@ func pktDecodeHead(e proto.Encoding, pkt *pktBuf) (connID uint32, msgCode uint16
 	}
 
 	if err != nil {
-		err = fmt.Errorf("decode header: %s", err)
+		err = fmt.Errorf("%c: decode header: %s", e, err)
 	}
 	return connID, msgCode, payload, err
 }