fix: false uninitialized read warning

Add asserts on file system reads to make sure
no positive values are returned, which would
make assumptions on error checks invalid.

This fixes clang tidy warnings on uninitialized
reads in uses of lfs_dir_get where only negative
returns are considered errors.

lfs.c

@@ -739,6 +739,7 @@ static lfs_stag_t lfs_dir_getslice(lfs_t *lfs, const lfs_mdir_t *dir,
         int err = lfs_bd_read(lfs,
                 NULL, &lfs->rcache, sizeof(ntag),
                 dir->pair[0], off, &ntag, sizeof(ntag));
+        LFS_ASSERT(err <= 0);
         if (err) {
             return err;
         }
@@ -767,6 +768,7 @@ static lfs_stag_t lfs_dir_getslice(lfs_t *lfs, const lfs_mdir_t *dir,
             err = lfs_bd_read(lfs,
                     NULL, &lfs->rcache, diff,
                     dir->pair[0], off+sizeof(tag)+goff, gbuffer, diff);
+            LFS_ASSERT(err <= 0);
             if (err) {
                 return err;
             }