Revert "util/IntrusiveList: allow the last disposer to destroy the IntrusiveList"

This reverts commit 669cbcd25a
("util/IntrusiveList: allow the last disposer to destroy the
IntrusiveList").  It was bad because it could lead to off-by-one crash
bugs when the last item was removed inside the previous item's
disposer.

We need a different solution for the other crash bug that was fixed by
the reverted commit.

src/util/IntrusiveList.hxx

@@ -293,20 +293,12 @@ public:
 
 	/**
 	 * Like clear(), but invoke a disposer function on each item.
+	 *
+	 * The disposer is not allowed to destruct the list.
 	 */
 	void clear_and_dispose(Disposer<value_type> auto disposer) noexcept {
-		bool is_empty = empty();
-
-		while (!is_empty) {
-			auto *item = &pop_front();
-
-			/* by checking empty() before invoking the
-			   disposer, it is possible for the disposer
-			   to destroy this IntrusiveList in the last
-			   call */
-			is_empty = empty();
-
-			disposer(item);
+		while (!empty()) {
+			disposer(&pop_front());
 		}
 	}