reject invalid offset/length in map storage

internal/storagetesting/map.go

@@ -2,6 +2,7 @@
 
 import (
 	"context"
+	"errors"
 	"sort"
 	"strings"
 	"sync"
@@ -28,6 +29,10 @@ func (s *mapStorage) GetBlock(ctx context.Context, id string, offset, length int
 			return data, nil
 		}
 
+		if int(offset) > len(data) || offset < 0 {
+			return nil, errors.New("invalid offset")
+		}
+
 		data = data[offset:]
 		if int(length) > len(data) {
 			return data, nil