erofs-utils: lib: fix small fragments again

`inode->fragment = NULL;` overrides `inode->fragmentoff`, which is
unintended.

In fact, the latest stress test [1] had already failed, but I didn't
notice it. Fedora openQA tests [2] also reported the issue.

[1] https://github.com/erofs/erofsstress/actions/runs/15881657361
[2] https://bodhi.fedoraproject.org/updates/FEDORA-2025-e6256ddcc4

Fixes: 9fa1a5838a2a ("erofs-utils: mkfs: fix memory leak from small fragments")
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20250626003219.3866538-1-hsiangkao@linux.alibaba.com
diff --git a/lib/fragments.c b/lib/fragments.c
index 6111631..3278f47 100644
--- a/lib/fragments.c
+++ b/lib/fragments.c
@@ -365,7 +365,6 @@
 		return 0;
 	}
 	inode->fragmentoff = (erofs_off_t)offset - len;
-	inode->fragment = NULL;
 	free(fi->data);
 	free(fi);
 	return 0;