plugin/dns64: Fixes a nil pointer dereference panic in dns64 during response (#8511)

This PR fixes a nil pointer dereference panic in dns64 during response,
when the internal A-record upstream re-lookup returns a nil response.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

plugin/dns64/dns64.go

@@ -6,6 +6,7 @@ package dns64
 import (
 	"context"
 	"errors"
+	"fmt"
 	"net"
 	"time"
 
@@ -122,6 +123,9 @@ func (d *DNS64) DoDNS64(ctx context.Context, w dns.ResponseWriter, r *dns.Msg, o
 	if err != nil {
 		return nil, err
 	}
+	if resp == nil {
+		return nil, fmt.Errorf("dns64: upstream returned no response")
+	}
 	out := d.Synthesize(r, origResponse, resp)
 	return out, nil
 }