fix: also assert inside lfs_bd_read

lfs.c

@@ -93,6 +93,7 @@ static int lfs_bd_read(lfs_t *lfs,
             // bypass cache?
             diff = lfs_aligndown(diff, lfs->cfg->read_size);
             int err = lfs->cfg->read(lfs->cfg, block, off, data, diff);
+            LFS_ASSERT(err <= 0);
             if (err) {
                 return err;
             }