[#805] pilorama: Fix TreeDrop

* If treeID is empty then deleting buckets for cursor may get
  invalidated. So, buckets should be gathered before deleting.

Signed-off-by: Airat Arifullin <a.arifullin@yadro.com>

pkg/local_object_storage/pilorama/boltdb.go

@@ -1115,11 +1115,12 @@ func (t *boltForest) TreeDrop(ctx context.Context, cid cidSDK.ID, treeID string)
 			c := tx.Cursor()
 			prefix := make([]byte, 32)
 			cid.Encode(prefix)
-			for k, _ := c.Seek(prefix); k != nil && bytes.HasPrefix(k, prefix); k, _ = c.Next() {
+			for k, _ := c.Seek(prefix); k != nil && bytes.HasPrefix(k, prefix); k, _ = c.Seek(prefix) {
 				err := tx.DeleteBucket(k)
 				if err != nil {
 					return err
 				}
+				_, _ = c.First() // rewind the cursor to the root page
 			}
 			return nil
 		}