[CountingPrivateSemaphore] Fix assert

As the code comment above the assert indicates, the atomic exchange
operation may either return nullptr or a valid Context pointer. And
this is what the assert() should assert. :)

Reported-by: Florian Fischer <florian.fl.fischer@fau.de>

emper/CountingPrivateSemaphore.cpp

 // SPDX-License-Identifier: LGPL-3.0-or-later
-// Copyright © 2020 Florian Schmaus
+// Copyright © 2020-2021 Florian Schmaus
 #include "CountingPrivateSemaphore.hpp"
 
 #include <cassert>	// for assert
@@ -46,7 +46,7 @@ auto CountingPrivateSemaphore::signalInternal() -> Context* {
 		// returns nullptr. In this case the block() function will
 		// have won the race.
 		Context* context = blockedContext.exchange(nullptr);
-		assert(context > (Context*)4096);
+		assert(!context || context > (Context*)4096);
 		return context;
 	}