Don't start HAP server without any devices

This ensures that when iOS reconnects, it doesn't find an empty server
and thinks all the devices have been removed, and removes them all
locally.

cmd/hapz2m/main.go

@@ -123,6 +123,10 @@ func main() {
 	}()
 
 	br.WaitConfigured()
+	if br.NumDevices() == 0 {
+		log.Println("No devices added to bridge. Refusing to start.")
+		return
+	}
 
 	log.Println("hapz2m configured. starting HAP server...")