Update CCTextureCache.cpp

Reset s_nAsyncRefCount to 0 when create CCTextureCache instance.
When use CCTextureCache::addImageAsync(), if CCTextureCache::purgeSharedTextureCache() is called before all image loaded, the s_nAsyncRefCount is bigger than 0, which cause the below code cannot run.

if (0 == s_nAsyncRefCount)
{
	CCDirector::sharedDirector()->getScheduler()->scheduleSelector(schedule_selector(CCTextureCache::addImageAsyncCallBack), this, 0, false);
}