Handle multicast flows

Change-Id: I00bbae880ae3976693e5d5fa08d5e2909f85bef6
diff --git a/internal/bbsim/devices/olt.go b/internal/bbsim/devices/olt.go
index d08d934..0d64a14 100644
--- a/internal/bbsim/devices/olt.go
+++ b/internal/bbsim/devices/olt.go
@@ -944,6 +944,10 @@
 		oltLogger.WithFields(log.Fields{
 			"FlowId": flow.FlowId,
 		}).Debugf("This is an OLT flow")
+	} else if flow.FlowType == "multicast" {
+		oltLogger.WithFields(log.Fields{
+			"FlowId": flow.FlowId,
+		}).Debugf("This is a multicast flow")
 	} else {
 		pon, err := o.GetPonById(uint32(flow.AccessIntfId))
 		if err != nil {
@@ -960,6 +964,7 @@
 				"IntfId": flow.AccessIntfId,
 				"err":    err,
 			}).Error("Can't find Onu")
+			return nil, err
 		}
 		if !o.enablePerf {
 			onu.Flows = append(onu.Flows, flowKey)