No routing/sending destination for ZeroIPString

pinging 0.0.0.0 should not go anywhere, but previously
it could be routed anywhere by default gateways etc.

Scenario: Level 3 It's dead Jim
1.) set the gateway for PC2 to router2
2.) ping net_switch1
The ping travels halfway around the network before it is dropped.

Even worse is L4 Who done it, where
originally the puzzle could be solved by
just pinging 0.0.0.0 enough times.

EduNetworkBuilder/NetworkDevice.cs

@@ -2832,6 +2832,11 @@ namespace EduNetworkBuilder
             NetworkCard tCard = null;
             NB_IPAddress DestAddr;
             string DHCPGW = "";
+
+            //Do not ping the zero string.  Should not go anywhere
+            if (dest != null && dest.GetIPString == NB.ZeroIPString)
+                return new NB_IPAddress(NB.ZeroIPString);
+
             foreach ( NB_IPAddress ip in RouteTable)
             {
                 if (ip.IsLocal(dest))