Exempt OleConvertIStorageToOLESTREAM / OleConvertIStorageToOLESTREAMEx from returning OLESTREAM

[Ben-Lichtman]

Summary

With these functions it's customary to write the OLESTREAM and its associated vtable yourself. The current API prohibits this by writing to a zeroed instance of OLESTREAM (causing a nullptr deref) without giving the user the opportunity to fill out the vtable / struct contents.