db/update/Queue: work around GCC7 -Wuninitialized

src/db/update/Queue.hxx

@@ -49,6 +49,10 @@ struct UpdateQueueItem {
 	bool IsDefined() const {
 		return id != 0;
 	}
+
+	void Clear() {
+		id = 0;
+	}
 };
 
 class UpdateQueue {

src/db/update/Service.cxx

@@ -252,7 +252,7 @@ UpdateService::RunDeferred()
 	delete walk;
 	walk = nullptr;
 
-	next = UpdateQueueItem();
+	next.Clear();
 
 	idle_add(IDLE_UPDATE);