refs.c: spell NULL pointer as NULL

A call to update_ref_lock() passes '0' to the 'int *type_p' parameter.
Noticed by sparse.  ("Using plain integer as NULL pointer")

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>

refs.c

@@ -3235,7 +3235,7 @@ int update_ref(const char *action, const char *refname,
 	       int flags, enum action_on_err onerr)
 {
 	struct ref_lock *lock;
-	lock = update_ref_lock(refname, oldval, flags, 0, onerr);
+	lock = update_ref_lock(refname, oldval, flags, NULL, onerr);
 	if (!lock)
 		return 1;
 	return update_ref_write(action, refname, sha1, lock, onerr);