tdf#91574: SLIDESHOW: crash when second monitor connected

0  0x00002aaab1f6a803 in VirtualDevice::InnerImplSetOutputSizePixel(Size const&, bool, boost::shared_array<unsigned char> const&, bool) (this=0x0, rNewSize=Size = {...}, bErase=true, pBuffer=..., bTopDown=false) at /home/julien/compile-libreoffice/libreoffice/vcl/source/gdi/virdev.cxx:293
1  0x00002aaab1f6ae54 in VirtualDevice::ImplSetOutputSizePixel(Size const&, bool, boost::shared_array<unsigned char> const&, bool) (this=0x0, rNewSize=Size = {...}, bErase=true, pBuffer=..., bTopDown=false) at /home/julien/compile-libreoffice/libreoffice/vcl/source/gdi/virdev.cxx:396
2  0x00002aaab1f6b1af in VirtualDevice::SetOutputSizePixel(Size const&, bool) (this=0x0, rNewSize=Size = {...}, bErase=true)
    at /home/julien/compile-libreoffice/libreoffice/vcl/source/gdi/virdev.cxx:444
3  0x00002aaae650e30f in vclcanvas::SpriteCanvasHelper::updateScreen(bool, bool&) (this=0x69daf50, bUpdateAll=true, io_bSurfaceDirty=@0x69dafe0: true)
    at /home/julien/compile-libreoffice/libreoffice/canvas/source/vcl/spritecanvashelper.cxx:316

So initialize maVDev

Change-Id: I4a79ba6e8f13c4a2bed3127742a9a9c928e133cd
Reviewed-on: https://gerrit.libreoffice.org/16770Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Michael Meeks <michael.meeks@collabora.com>
Tested-by: Michael Meeks <michael.meeks@collabora.com>

canvas/source/vcl/spritecanvashelper.cxx

@@ -177,7 +177,7 @@ namespace vclcanvas
     SpriteCanvasHelper::SpriteCanvasHelper() :
         mpRedrawManager( NULL ),
         mpOwningSpriteCanvas( NULL ),
-        maVDev(),
+        maVDev(VclPtr<VirtualDevice>::Create()),
         maLastUpdate(),
         mbShowFrameInfo( false ),
         mbShowSpriteBounds( false ),