Clear the user cache, when re-using an existing contact point in the contact cache (btPersistentManifold).

Issues reported here: http://www.bulletphysics.com/Bullet/phpBB3/viewtopic.php?f=9&t=2257

src/BulletCollision/NarrowPhaseCollision/btPersistentManifold.cpp

@@ -175,7 +175,7 @@ void btPersistentManifold::AddManifoldPoint(const btManifoldPoint& newPoint)
 #else
 		insertIndex = 0;
 #endif
-
+		clearUserCache(m_pointCache[insertIndex]);
 		
 	} else
 	{