input-device: Guard against double free

Dispose() may be called more than once, so calling g_free directly
on the device name is unsafe. Instead, use g_clear_pointer() to
make sure we don't attempt to free the memory again.

https://bugzilla.gnome.org/show_bug.cgi?id=719563

clutter/clutter-input-device.c

@@ -79,7 +79,7 @@ clutter_input_device_dispose (GObject *gobject)
 {
   ClutterInputDevice *device = CLUTTER_INPUT_DEVICE (gobject);
 
-  g_free (device->device_name);
+  g_clear_pointer (&device->device_name, g_free);
 
   if (device->associated != NULL)
     {