add uc_tailq_link() and use it in uc_tailq_remove()

include/ucore/tailq.h

@@ -85,13 +85,20 @@ static UC_INLINE void uc_tailq_insert_head(struct TailQ *head, struct TailQ *ent
     uc_tailq_insert(entry, head, head->next);
 }
 
+/** Link the first and second entry together,
+ */
+static UC_INLINE void uc_tailq_link(struct TailQ *first, struct TailQ *second)
+{
+    first->next = second;
+    second->prev = first;
+}
+
 /** Remove the entry from the list it is part of.
  * (It *must* already be part of a list.
  */
 static UC_INLINE void uc_tailq_remove(struct TailQ *entry)
 {
-    entry->prev->next = entry->next;
+    uc_tailq_link(entry->prev, entry->next);
-    entry->next->prev = entry->prev;
 
 //help catch illegal use:
     entry->next = (void*)0x101;