p2p/discover: bump failure counter only if no nodes were provided (#19362)

This resolves a minor issue where neighbors responses containing less
than 16 nodes would bump the failure counter, removing the node. One
situation where this can happen is a private deployment where the total
number of extant nodes is less than 16.

Issue found by @jsying.

p2p/discover/table.go

@@ -313,7 +313,7 @@ func (tab *Table) findnode(n *node, targetKey encPubkey, reply chan<- []*node) {
 		// Avoid recording failures on shutdown.
 		reply <- nil
 		return
-	} else if err != nil || len(r) == 0 {
+	} else if len(r) == 0 {
 		fails++
 		tab.db.UpdateFindFails(n.ID(), n.IP(), fails)
 		log.Trace("Findnode failed", "id", n.ID(), "failcount", fails, "err", err)