DoublyLinkedList: Rework Remove() function.

* Check fFirst/fLast instead of previous/next. Avoids
   list corruption when trying to remove already-removed
   elements, instead will cause null-dereference KDL.

 * Always set next/previous to NULL even when DEBUG is not
   enabled.

headers/private/kernel/util/DoublyLinkedList.h

@@ -505,7 +505,9 @@ DOUBLY_LINKED_LIST_TEMPLATE_LIST
 void
 DOUBLY_LINKED_LIST_CLASS_NAME::Remove(Element* element)
 {
-	if (element) {
+	if (element == NULL)
+		return;
+
 #if DEBUG_DOUBLY_LINKED_LIST
 	ASSERT_PRINT(fFirst != NULL && fLast != NULL
 		&& (fFirst != fLast || element == fFirst),
@@ -513,19 +515,18 @@ DOUBLY_LINKED_LIST_CLASS_NAME::Remove(Element* element)
 #endif
 
 	Link* elLink = sGetLink(element);
-		if (elLink->previous)
-			sGetLink(elLink->previous)->next = elLink->next;
-		else
-			fFirst = elLink->next;
-		if (elLink->next)
-			sGetLink(elLink->next)->previous = elLink->previous;
-		else
-			fLast = elLink->previous;
 
-#if DEBUG_DOUBLY_LINKED_LIST
+	if (element == fFirst)
+		fFirst = elLink->next;
+	else
+		sGetLink(elLink->previous)->next = elLink->next;
+
+	if (element == fLast)
+		fLast = elLink->previous;
+	else
+		sGetLink(elLink->next)->previous = elLink->previous;
+
 	elLink->next = elLink->previous = NULL;
-#endif
-	}
 }
 
 // Swap