Resolve js_DestroyContext assertion during reycle of event_thread caused by

change in rev 1.507 (Dec-20-2008):
sbbs->event_thread_running was being set to false *before* js_cleanup() was
being called, so it was possible for the js_cleanup() in the sbbs_t destructor
to be called simultaneously with the js_cleanup() at the end of the
event_thread causing mayhem.