runtime/network: Fix error reporting in listRoutes()

If the upcast from resultingRoutes to *grpc.IRoutes fails, we return
(nil, err), but previous code ensures that err is nil at that point, so we
return no error.

fixes #1206

Forward port of
0ffaeeb5d8

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

src/runtime/virtcontainers/kata_agent.go

@@ -648,10 +648,10 @@ func (k *kataAgent) listRoutes() ([]*pbTypes.Route, error) {
 		return nil, err
 	}
 	resultRoutes, ok := resultingRoutes.(*grpc.Routes)
-	if ok {
-		return resultRoutes.Routes, err
+	if !ok {
+		return nil, fmt.Errorf("Unexpected type %T for routes", resultingRoutes)
 	}
-	return nil, err
+	return resultRoutes.Routes, nil
 }
 
 func (k *kataAgent) getAgentURL() (string, error) {