ares_init_options() fix crash on misuse of event thread options

If an invalid event thread system was provided, it would crash during cleanup due to a NULL pointer dereference.

Fixes Issue: #749
Fix By: Brad House (@bradh352)

src/lib/ares_event_thread.c

@@ -309,7 +309,7 @@ static void ares_event_thread_destroy_int(ares_event_thread_t *e)
   ares__htable_asvp_destroy(e->ev_handles);
   e->ev_handles = NULL;
 
-  if (e->ev_sys->destroy) {
+  if (e->ev_sys && e->ev_sys->destroy) {
     e->ev_sys->destroy(e);
   }