prevent access to dangling pointers when handling libvlc_event
Currently libvlc_event_detach() is done with -(void)dealloc, and since the instance is released at that point, the timing to calllibvlc_event_detach ()is late.
Therefore, it is possible that libvlc_event_detach() will not be in time and will send libvlc_callback to the object that has already been released.
This is an attempt to call libvlc_event_detach() before -(void)dealloc to prevent it.