BGE: Fix issues with async libload.

This patch fixes:
- the call of LibFree on a unfinished loaded library;
- memory leak created on end of game : the async libraries are loaded but not converted, so not freed with the master scene.

Reviewers: campbellbarton, sybren, youle, hg1, moguri, lordloki

Reviewed By: moguri, lordloki

Differential Revision: https://developer.blender.org/D1571

source/gameengine/Ketsji/KX_KetsjiEngine.cpp

@@ -1273,6 +1273,7 @@ void KX_KetsjiEngine::StopEngine()
 {
 	if (m_bInitialized)
 	{
+		m_sceneconverter->FinalizeAsyncLoads();
 
 		if (m_animation_record)
 		{