be_pair: release shared lock with the latest of bufferevent_pair

be_pair: release shared lock with the latest of bufferevent_pair

Then next code sample will use free'd lock:
  evthread_use_pthreads();
  ...
  assert(!bufferevent_pair_new(base, BEV_OPT_THREADSAFE, pair));
  ...
  bufferevent_free(pair[0]); # refcnt == 0 -> unlink
  bufferevent_free(pair[1]); # refcnt == 0 -> unlink
  ...
  event_base_free() -> finalizers -> EVTHREAD_FREE_LOCK(bev1->lock)
                                  -> BEV_LOCK(bev2->lock) <-- *already freed*

While if you will reverse the order:
  bufferevent_free(pair[1]); # refcnt == 0 -> unlink
  bufferevent_free(pair[0]); # refcnt == 0 -> unlink
  ...
  event_base_free() -> finalizers -> BEV_LOCK(bev2->lock)/!own_lock/BEV_UNLOCK(bev2->lock)
                                  -> EVTHREAD_FREE_LOCK(bev1->lock) (own_lock)

It is ok now, but I guess that it will be better to relax order of
freeing pairs.