go/protocol: fix out of order tag detection

out of order tag detection wasn't happening because the previous tag
value wasn't being set correctly in the decode loop

Change-Id: I060d4b15312cd897a1c2d15d4c8b917c2df30128

go/protocol/protocol.go

diff --git a/go/protocol/protocol.go b/go/protocol/protocol.go
index a1f27b1..88c4298 100644
--- a/go/protocol/protocol.go
+++ b/go/protocol/protocol.go
@@ -201,6 +201,7 @@
 		payloads = payloads[length:]
 		ret[tag] = payload
 		currentOffset = nextOffset
+		lastTag = tag
 	}
 
 	return ret, nil