go/neo/neonet: Increase signal/noise in dump of packets with decode problems

We recently hit "decode: buffer overflow" errors due to mismatch in
between NEO/go and NEO/py (see previous patch). With dumpio=true that
problem was showing itself as

    127.0.0.1:50624 > 127.0.0.1:41863: .5 GetObject &{0000000000000000 ffffffffffffffff 0285cbac258bf266}
    127.0.0.1:50624 < 127.0.0.1:41863: .5 (proto.Error) decode: buffer overflow; #24 [24]: 00 00 00 03 00 00 00 10 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 Error ACK ;  [24]tail: 00 00 00 03 00 00 00 10 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30
        xtesting.go:306: load 0285cbac258bf265:0000000000000000: returned err unexpected:
            have: neo://1@127.0.0.1:24078: load 0285cbac258bf265:0000000000000000: 127.0.0.1:50624 - 127.0.0.1:41863 .5: decode: decode: buffer overflow
            want: neo://1@127.0.0.1:24078: load 0285cbac258bf265:0000000000000000: 0000000000000000: object was not yet created

Here after printing error and dumping all bytes from packet payload, it
was printing again message type, message value from to-be-decoded place
(which is zero-initialized) and data bytes again.
-> Fix it not to print anything after dump of payload data:

    127.0.0.1:60518 > 127.0.0.1:46719: .5 GetObject &{0000000000000000 ffffffffffffffff 0285cbac258bf266}
    127.0.0.1:60518 < 127.0.0.1:46719: .5 (Error) decode: buffer overflow; #24 [24]: 00 00 00 03 00 00 00 10 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30
        xtesting.go:306: load 0285cbac258bf265:0000000000000000: returned err unexpected:
            have: neo://1@127.0.0.1:27853: load 0285cbac258bf265:0000000000000000: 127.0.0.1:60518 - 127.0.0.1:46719 .5: decode: decode: buffer overflow
            want: neo://1@127.0.0.1:27853: load 0285cbac258bf265:0000000000000000: 0000000000000000: object was not yet created

go/neo/neonet/pkt.go

@@ -100,14 +100,14 @@ func (pkt *pktBuf) String() string {
 	msg := reflect.New(msgType).Interface().(proto.Msg)
 	n, err := msg.NEOMsgDecode(data)
 	if err != nil {
-		s += fmt.Sprintf(" (%s) %v; #%d [%d]: % x", msgType, err, msgLen, len(data), data)
-	}
-
-	s += fmt.Sprintf(" %s %v", msgType.Name(), msg) // XXX or %+v better?
-
-	if n < len(data) {
-		tail := data[n:]
-		s += fmt.Sprintf(" ;  [%d]tail: % x", len(tail), tail)
+		s += fmt.Sprintf(" (%s) %v; #%d [%d]: % x", msgType.Name(), err, msgLen, len(data), data)
+	} else {
+		s += fmt.Sprintf(" %s %v", msgType.Name(), msg) // XXX or %+v better?
+
+		if n < len(data) {
+			tail := data[n:]
+			s += fmt.Sprintf(" ;  [%d]tail: % x", len(tail), tail)
+		}
 	}
 
 	return s