go/neo/neonet: Fix thinko in recvPkt

We were returning packet with tail read for next packet data.
Previously decoding was forgiving, but upcoming rework for msgpack
support will add check to catch packets with overlong payload, 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

-// Copyright (C) 2016-2020  Nexedi SA and Contributors.
+// Copyright (C) 2016-2021  Nexedi SA and Contributors.
 //                          Kirill Smelkov <kirr@nexedi.com>
 //
 // This program is free software: you can Use, Study, Modify and Redistribute
@@ -1229,7 +1229,7 @@ func (nl *NodeLink) recvPkt() (*pktBuf, error) {
 	}
 
 	// fixup data/pkt
-	data = data[:n]
+	data = data[:pktLen]
 	pkt.data = data
 
 	if dumpio {