The maximum size of the name returned by Stat() on a top-level storage has
nothing to do with DIRENTRY_NAME_BUFFER_LEN. Windows can return longer names
and probably has a limit of MAX_PATH. Also, Stat always returns an absolute
pathname, so we don't need a special case in StgOpenStorage.

Fixes stack corruption in the 64-bit tests. Thanks to Paul Vriens for debugging this.

This makes it consistent with dxgi_adapter.

This fixes a regression introduced by 2b2fc827.

Found by Valgrind.