Fix: pacemaker-attrd: prevent segfault if a peer leaves when its name is unknown yet

If nodes are not configured with names in corosync, when a node joins
the cluster, it takes a while for it to learn the names of all the other
online peers. Previously, if any of the peers happened to leave when its
name was unknown to us yet, pacemaker-attrd would trigger assertion in
attrd_peer_remove() and segfault in
attrd_remove_peer_protocol_ver().