thread/WindowsFuture: remove wrong address_of operator

NEWS

 ver 0.23.14 (not yet released)
 * decoder
   - mad: fix calculation of LAME peak values
+* mixer
+  - wasapi: fix problem setting volume
 
 ver 0.23.13 (2023/05/22)
 * input

src/thread/WindowsFuture.hxx

@@ -130,7 +130,7 @@ public:
 
 	void set_value(const T &value) {
 		std::unique_lock<CriticalSection> lock(mutex);
-		if (!std::holds_alternative<std::monostate>(&result)) {
+		if (!std::holds_alternative<std::monostate>(result)) {
 			throw WinFutureError(WinFutureErrc::promise_already_satisfied);
 		}
 		result.template emplace<T>(value);