use current height as default height

src/neo/Consensus/ConsensusContext.cs

@@ -399,7 +399,7 @@ public void Reset(byte viewNumber)
                         if (previous_last_seen_message.TryGetValue(validator, out int value))
                             LastSeenMessage[validator] = value;
                         else
-                            LastSeenMessage[validator] = -1;
+                            LastSeenMessage[validator] = (int)Snapshot.Height;
                     }
                 }
                 keyPair = null;