MINOR: queue: make sure pendconn->strm->pend_pos is always valid

MINOR: queue: make sure pendconn->strm->pend_pos is always valid

pendconn_add() used to assign strm->pend_pos very late, after unlocking
the queue, so that a watching thread could see a random value in
pendconn->strm->pend_pos even while holding the lock on the element and
the queue itself. While there's currently nothing wrong with this, it
costs nothing to arrange it and will simplify code analysis later.