devices: add maybeOneDevice()

cmd/fdroidcl/devices.go

@@ -40,14 +40,25 @@ func startAdbIfNeeded() {
 	}
 }
 
-func mustOneDevice() *adb.Device {
+func maybeOneDevice() *adb.Device {
 	startAdbIfNeeded()
 	devices, err := adb.Devices()
 	if err != nil {
 		log.Fatalf("Could not get devices: %v", err)
 	}
-	if len(devices) != 1 {
+	if len(devices) > 1 {
-		log.Fatalf("Exactly one connected device is needed")
+		log.Fatalf("At most one connected device can be used")
+	}
+	if len(devices) < 1 {
+		return nil
 	}
 	return devices[0]
 }
+
+func mustOneDevice() *adb.Device {
+	device := maybeOneDevice()
+	if device == nil {
+		log.Fatalf("A connected device is needed")
+	}
+	return device
+}