*: use NULL for null pointers

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/e2fsprogs/old_e2fsprogs/blkid/devname.c b/e2fsprogs/old_e2fsprogs/blkid/devname.c
index 348e5d4..fad92cb 100644
--- a/e2fsprogs/old_e2fsprogs/blkid/devname.c
+++ b/e2fsprogs/old_e2fsprogs/blkid/devname.c
@@ -258,7 +258,7 @@
 {
 	FILE *proc;
 	char line[1024];
-	char ptname0[128], ptname1[128], *ptname = 0;
+	char ptname0[128], ptname1[128], *ptname = NULL;
 	char *ptnames[2];
 	dev_t devs[2];
 	int ma, mi;