*: use NULL for null pointers

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/e2fsprogs/old_e2fsprogs/mke2fs.c b/e2fsprogs/old_e2fsprogs/mke2fs.c
index 32f0329..5203645 100644
--- a/e2fsprogs/old_e2fsprogs/mke2fs.c
+++ b/e2fsprogs/old_e2fsprogs/mke2fs.c
@@ -799,8 +799,8 @@
 	int		show_version_only = 0;
 	ext2_ino_t	num_inodes = 0;
 	errcode_t	retval;
-	char *		extended_opts = 0;
-	const char *	fs_type = 0;
+	char *		extended_opts = NULL;
+	const char *	fs_type = NULL;
 	blk_t		dev_size;
 	long		sysval;