LockSet:tryAcquireOrBackoff use optional<ref_wrapper<Qutex>>

Replace the previous Qutex** raw pointer with a std::optional.

include/lockSet.h

@@ -7,6 +7,7 @@
 #include <stdexcept>
 #include <utility>
 #include <memory>
+#include <optional>
 #include <qutex.h>
 #include <lockerAndInvokerBase.h>
 
@@ -85,7 +86,9 @@ public:
 	 * @return true if all locks were acquired, false otherwise
 	 */
 	bool tryAcquireOrBackOff(
-		LockerAndInvokerBase &lockvoker, Qutex **firstFailedQutex = nullptr);
+		LockerAndInvokerBase &lockvoker,
+		std::optional<std::reference_wrapper<Qutex>> &firstFailedQutex
+			= std::nullopt);
 	void unregisterFromQutexQueues();
 
 	// @brief Release all locks