Dereference device with a defer call to prevent memory leaks (#98)

We should always dereference devices. c.libusb.open is keeping its own internal reference so there is no risk of breaking anything.

libusb.go

@@ -209,7 +209,7 @@ func (libusbImpl) getDevices(ctx *libusbContext) ([]*libusbDevice, error) {
 	for _, d := range devs {
 		ret = append(ret, (*libusbDevice)(d))
 	}
-	// devices will be dereferenced later, during close.
+	// devices must be dereferenced by the caller to prevent memoryleaks
 	C.libusb_free_device_list(list, 0)
 	return ret, nil
 }

usb.go

@@ -187,16 +187,17 @@ func (c *Context) OpenDevices(opener func(desc *DeviceDesc) bool) ([]*Device, er
 	var ret []*Device
 	for _, dev := range list {
 		desc, err := c.libusb.getDeviceDesc(dev)
+		defer c.libusb.dereference(dev)
 		if err != nil {
-			c.libusb.dereference(dev)
 			reterr = err
 			continue
 		}
 
-		if opener(desc) {
+		if !opener(desc) {
+			continue
+		}
 		handle, err := c.libusb.open(dev)
 		if err != nil {
-				c.libusb.dereference(dev)
 			reterr = err
 			continue
 		}
@@ -205,9 +206,7 @@ func (c *Context) OpenDevices(opener func(desc *DeviceDesc) bool) ([]*Device, er
 		c.mu.Lock()
 		c.devices[o] = true
 		c.mu.Unlock()
-		} else {
-			c.libusb.dereference(dev)
-		}
+
 	}
 	return ret, reterr
 }