Regression from commit b160db92
(tdf#96947 vcl opengl win: fix background of menu items w/ check/radio
marks, 2018-05-14), the problem was that GetMenuPopupMarkRegion() did an
unconditional downcast from ImplControlValue to MenupopupValue.  This
looked safe, but when the extracted function was used in
WinSalGraphics::drawNativeControl(), then that argument was not safe to
downcast.

Fix the problem by switching to safe downcast, and returning an empty
rectangle (instead of a bogus one) on error.

This way the old bug stays fixed but fontwork popup menu works again as
well.

Change-Id: I2b69b9f2e6823850892ac73df78dac5c2f04ec03
Reviewed-on: https://gerrit.libreoffice.org/68883Reviewed-by: Miklos Vajna <vmiklos@collabora.com>
Tested-by: Jenkins