evdev: plug a potential memleak

For touchpads, device->dispatch is set up when exiting
evdev_handle_device() and a potential source for a memleak.
This can't actually happen at the moment, as evdev_handle_device() won't
fail for touchpads after setting up the dispatch but prevent this from
happening in the future.

src/evdev.c

@@ -593,6 +593,8 @@ evdev_device_create(struct weston_seat *seat, const char *path, int device_fd)
 	device->devname = strdup(devname);
 
 	if (!evdev_handle_device(device)) {
+		if (device->dispatch)
+			device->dispatch->interface->destroy(dispatch);
 		if (device->mtdev)
 			mtdev_close_delete(device->mtdev);
 		free(device->devnode);