fix divide by zero issue

network/gateway/gateway.go

@@ -23,10 +23,14 @@ type HostGetter interface {
 	GetHost(hostId *id.ID) (*connect.Host, bool)
 }
 
+// Get the Host of a random gateway in the NDF
 func Get(ndf *ndf.NetworkDefinition, hg HostGetter, rng io.Reader) (*connect.Host, error) {
-	// Get a random gateway
-	gateways := ndf.Gateways
-	gwIdx := ReadRangeUint32(0, uint32(len(gateways)), rng)
+	gwLen := uint32(len(ndf.Gateways))
+	if gwLen == 0 {
+		return nil, errors.Errorf("no gateways available")
+	}
+
+	gwIdx := ReadRangeUint32(0, gwLen, rng)
 	gwID, err := id.Unmarshal(ndf.Nodes[gwIdx].ID)
 	if err != nil {
 		return nil, errors.WithMessage(err, "failed to get Gateway")
@@ -40,6 +44,7 @@ func Get(ndf *ndf.NetworkDefinition, hg HostGetter, rng io.Reader) (*connect.Hos
 	return gwHost, nil
 }
 
+// Get the last gateway Host from the given RoundInfo
 func GetLast(hg HostGetter, ri *mixmessages.RoundInfo) (*connect.Host, error) {
 	roundTop := ri.GetTopology()
 	lastGw, err := id.Unmarshal(roundTop[len(roundTop)-1])