[ContextManager] Add assert(currentFiber)

emper/ContextManager.cpp

@@ -148,6 +148,7 @@ void ContextManager::discardAndResume(Context* context) {
 	// the end of its dispatch loop, and hence we need to ensure that
 	// its fiber is recycled.
 	AbstractFiber* currentFiber = contextToFree->getFiber();
+	assert(currentFiber);
 	runtime.dispatcher.recycle(currentFiber);
 
 	contextToFree->discardAndResume(context);