RTFSdrImport::resolveDhgt(): fix crashing C-cast

Inside DomainMapper::graphicZOrderHelper() this is offset by 0x40 so
whatever that cast does, it's doing it wrong.

Change-Id: Ie48467c12828137a521cba47c2e04cac65201d1d
(cherry picked from commit 8dcb56fdcea455012ef010c343ab036955678f4d)
Reviewed-on: https://gerrit.libreoffice.org/8222
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>