wal: save empty snapshot when create

So caller can open at empty snapshot to read all entries.

wal/wal_test.go

@@ -24,6 +24,7 @@ import (
 	"reflect"
 	"testing"
 
+	"github.com/coreos/etcd/pkg/pbutil"
 	"github.com/coreos/etcd/raft/raftpb"
 	"github.com/coreos/etcd/wal/walpb"
 )
@@ -58,6 +59,13 @@ func TestNew(t *testing.T) {
 	if err != nil {
 		t.Fatalf("err = %v, want nil", err)
 	}
+	r := &walpb.Record{
+		Type: snapshotType,
+		Data: pbutil.MustMarshal(&walpb.Snapshot{}),
+	}
+	if err = e.encode(r); err != nil {
+		t.Fatalf("err = %v, want nil", err)
+	}
 	e.flush()
 	if !reflect.DeepEqual(gd, wb.Bytes()) {
 		t.Errorf("data = %v, want %v", gd, wb.Bytes())