[AD] 4.9 shutdown bug

[Peter Wang <novalazy@xxxxxxxxxx>]

_al_exit() calls al_set_current_display(NULL);

void al_set_current_display(ALLEGRO_DISPLAY *display)
{  
   if (display)
      display->vt->set_current_display(display);
      
   if ((tls = tls_get()) == NULL)
      return;
   tls->current_display = display;

   al_set_target_bitmap(al_get_backbuffer());
}

But at the time al_get_backbuffer() is called, _al_current_display will
be NULL so it crashes.  It wasn't obvious to me how it's intended to
work so I leave it to someone else :)

Peter