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>
(cherry picked from commit 84e8e9f178308f082b0c51c3e33655e3a082c057)
Reviewed-on: https://gerrit.libreoffice.org/8226
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>
Reviewed-by: Björn Michaelsen <bjoern.michaelsen@canonical.com>
Reviewed-by: Eike Rathke <erack@redhat.com>