Core/SAI: Fix assertion triggered in SmartScript

Remove assertion in SmartScript added in b8625f41bd6131e7fb3ff126780da18d5d2959fd and handle SmartScripts with GetBaseObject() returning NULL , like SmartTrigger . In this case SmartScript::mTargetStorage cannot be sanitized and might contain references to free'd WorldObjects if the script doesn't last a single Map::Update() call.