ofz#4055 Integer-overflow

Change-Id: I15f0a48c1e0c4c03f99a74d4d9b58e064d108c72
Reviewed-on: https://gerrit.libreoffice.org/44304Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

emfio/source/reader/mtftools.cxx

@@ -2004,9 +2004,10 @@ namespace emfio
         {
             if ( mnMapMode == MM_ISOTROPIC ) //TODO: WHAT ABOUT ANISOTROPIC???
             {
-                Size aSize( (mnWinExtX + mnWinOrgX) >> MS_FIXPOINT_BITCOUNT_28_4,
-                          -((mnWinExtY - mnWinOrgY) >> MS_FIXPOINT_BITCOUNT_28_4));
-
+                sal_Int32 nX, nY;
+                if (o3tl::checked_add(mnWinExtX, mnWinOrgX, nX) || o3tl::checked_sub(mnWinExtY, mnWinOrgY, nY))
+                    return;
+                Size aSize(nX >> MS_FIXPOINT_BITCOUNT_28_4, -(nY >> MS_FIXPOINT_BITCOUNT_28_4));
                 SetDevExt(aSize, false);
             }
         }