fix nil piece

server/torr/storage/torrstor/cache.go

@@ -83,7 +83,7 @@ func (c *Cache) Piece(m metainfo.Piece) storage.PieceImpl {
 	if val, ok := c.pieces[m.Index()]; ok {
 		return val
 	}
-	return nil
+	return &PieceFake{}
 }
 
 func (c *Cache) Close() error {

server/torr/storage/torrstor/piecefake.go

@@ -0,0 +1,33 @@
+package torrstor
+
+import (
+	"errors"
+	"github.com/anacrolix/torrent/storage"
+)
+
+type PieceFake struct{}
+
+func (PieceFake) ReadAt(p []byte, off int64) (n int, err error) {
+	err = errors.New("fake")
+	return
+}
+
+func (PieceFake) WriteAt(p []byte, off int64) (n int, err error) {
+	err = errors.New("fake")
+	return
+}
+
+func (PieceFake) MarkComplete() error {
+	return errors.New("fake")
+}
+
+func (PieceFake) MarkNotComplete() error {
+	return errors.New("fake")
+}
+
+func (PieceFake) Completion() storage.Completion {
+	return storage.Completion{
+		Complete: false,
+		Ok:       true,
+	}
+}