When closing a filtering bufferevent, clear callbacks on the underlying bufferevent

When closing a filtering bufferevent, clear callbacks on the underlying bufferevent

Previously, if BEV_OPT_CLOSE_ON_FREE wasn't set on a
bufferevent_filter or a filtering bufferevent_openssl, when we went
to free the filtering bufferevent, we'd leave the underlying
bufferevent unchanged.  That's not so good, since the callbacks are
set to activate stuff in the filtering bufferevent that we're about
to free.  Instead, set all the callbacks to NULL.