We still don't return a SalGraphics object from AcquireGraphics
without a valid DC. But internally we keep the WinSalGraphics
objects around, so we now have to verify the DC before using it.

In the end this also fixes the leak of the threaded SalGraphics
of the frame.

Change-Id: I267c96c04b7d00cb66a6c84c63d1373ebe0f529f
Reviewed-on: https://gerrit.libreoffice.org/50908Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Jan-Marek Glogowski <glogow@fbihome.de>