Fix panic when backup bootstrap peer load and save funcs are nil

A panic occurs when the first bootstrap round runs is these functions are not assigned in the configuration:
- `LoadBackupBootstrapPeers`
- `SaveBackupBootstrapPeers`

This fix assumes that it is acceptable for these functions to be nil, as it may be desirable to disable the bakcup peer load and save functionality.

core/bootstrap/bootstrap.go

@@ -125,7 +125,9 @@ func Bootstrap(id peer.ID, host host.Host, rt routing.Routing, cfg BootstrapConf
 	doneWithRound <- struct{}{}
 	close(doneWithRound) // it no longer blocks periodic
 
-	startSavePeersAsTemporaryBootstrapProc(cfg, host, proc)
+	if cfg.LoadBackupBootstrapPeers != nil && cfg.SaveBackupBootstrapPeers != nil {
+		startSavePeersAsTemporaryBootstrapProc(cfg, host, proc)
+	}
 
 	return proc, nil
 }
@@ -243,6 +245,11 @@ func bootstrapRound(ctx context.Context, host host.Host, cfg BootstrapConfig) er
 		}
 	}
 
+	if cfg.LoadBackupBootstrapPeers == nil {
+		log.Debugf("not enough bootstrap peers to fill the remaining target of %d connections", numToDial)
+		return ErrNotEnoughBootstrapPeers
+	}
+
 	log.Debugf("not enough bootstrap peers to fill the remaining target of %d connections, trying backup list", numToDial)
 
 	tempBootstrapPeers := cfg.LoadBackupBootstrapPeers(ctx)

core/bootstrap/bootstrap_test.go

@@ -1,8 +1,12 @@
 package bootstrap
 
 import (
+	"crypto/rand"
 	"testing"
+	"time"
 
+	"github.com/libp2p/go-libp2p"
+	"github.com/libp2p/go-libp2p/core/crypto"
 	"github.com/libp2p/go-libp2p/core/peer"
 	"github.com/libp2p/go-libp2p/core/test"
 )
@@ -23,3 +27,31 @@ func TestRandomizeAddressList(t *testing.T) {
 		t.Fail()
 	}
 }
+
+func TestNoTempPeersLoadAndSave(t *testing.T) {
+	period := 500 * time.Millisecond
+	bootCfg := BootstrapConfigWithPeers(nil)
+	bootCfg.MinPeerThreshold = 2
+	bootCfg.Period = period
+
+	priv, pub, err := crypto.GenerateEd25519Key(rand.Reader)
+	if err != nil {
+		t.Fatal(err)
+	}
+	peerID, err := peer.IDFromPublicKey(pub)
+	if err != nil {
+		t.Fatal(err)
+	}
+	p2pHost, err := libp2p.New(libp2p.Identity(priv))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	bootstrapper, err := Bootstrap(peerID, p2pHost, nil, bootCfg)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	time.Sleep(4 * period)
+	bootstrapper.Close()
+}