util: add an extra assert for list_insert()

If we're adding an element that's not null or not a freshly initialized list,
chances are we haven't removed it from a previous list.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Jonas Ådahl <jadahl@gmail.com>

src/libinput-util.c

@@ -52,6 +52,8 @@ list_insert(struct list *list, struct list *elm)
 {
 	assert((list->next != NULL && list->prev != NULL) ||
 	       !"list->next|prev is NULL, possibly missing list_init()");
+	assert(((elm->next == NULL && elm->prev == NULL) || list_empty(elm)) ||
+	       !"elm->next|prev is not NULL, list node used twice?");
 
 	elm->prev = list;
 	elm->next = list->next;