util/IntrusiveList: add missing initializer to insert_after()

We must not call std::next() if the list head was not yet initialized.
This was missing in commit 7065425927

src/util/IntrusiveList.hxx

@@ -538,6 +538,10 @@ public:
 	 * Like insert(), but insert after the given position.
 	 */
 	void insert_after(iterator p, reference t) noexcept {
+		if constexpr (options.zero_initialized)
+			if (head.next == nullptr)
+				head = {&head, &head};
+
 		insert(std::next(p), t);
 	}