Fix false fatal err while reading peers in config (#19)

Earlier we didn't differ is variable empty or just not set, that's why
when we met empty address we just finished reading of the list of peers
and printed "skip, empty address" anyway:
```
if address == "" {
	a.log.Warn("skip, empty address")
	break
}
```

Changes in the last commit added check if `peers.i.address` is empty and
if true, raise fatal. It led to false error when the list of peers is
over.

cmd/neofs-send-authz/app.go

@@ -111,7 +111,11 @@ func (a *app) initPool(ctx context.Context, key *keys.PrivateKey) {
 	p.SetClientRebalanceInterval(rebalanceInterval)
 
 	for i := 0; ; i++ {
-		key := cfgPeers + "." + strconv.Itoa(i) + "."
+		key := cfgPeers + "." + strconv.Itoa(i)
+		if !a.cfg.IsSet(key) {
+			break
+		}
+		key += "."
 		address := a.cfg.GetString(key + "address")
 		weight := a.cfg.GetFloat64(key + "weight")
 		priority := a.cfg.GetInt(key + "priority")