Fix being able to enable/disable multicast

src/yggdrasil/multicast.go

@@ -16,6 +16,10 @@ type multicast struct {
 func (m *multicast) init(core *Core) {
 	m.core = core
 	m.groupAddr = "[ff02::114]:9001"
+	// Check if we've been given any expressions
+	if len(m.core.ifceExpr) == 0 {
+		return
+	}
 	// Ask the system for network interfaces
 	allifaces, err := net.Interfaces()
 	if err != nil {