fs/squashfs: fix sqfs_read_sblk()

Setting sblk = NULL has no effect on the caller.
We want to set *sblk = NULL if an error occurrs to avoid usage after free.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/fs/squashfs/sqfs.c b/fs/squashfs/sqfs.c
index 547d2fd..90bf32c 100644
--- a/fs/squashfs/sqfs.c
+++ b/fs/squashfs/sqfs.c
@@ -49,7 +49,7 @@
 
 	if (sqfs_disk_read(0, 1, *sblk) != 1) {
 		free(*sblk);
-		sblk = NULL;
+		*sblk = NULL;
 		return -EINVAL;
 	}