fix spell mistake

null To NULL

Author: rovast

c-cpp/06_linkedlist/single_list.c

@@ -47,7 +47,7 @@ struct single_list* del(struct single_list **prev)
 
 	if (!prev)
 		return NULL;
-	if (*prev == null)
+	if (*prev == NULL)
 		return NULL;
 	tmp = *prev;
 	*prev = (*prev)->next;