mbCurVisible is immediately read in ImplPrepForDraw, which inits some more
members but not mnStyle which is then read in ImplCursorInvert.  Regression
introduced with b9c10ee9 "weld
ClassificationDialog", found when valgrind'ing UITest_classification.

Change-Id: I986af0531e250953b8e45c914997b31c53a9ba91
Reviewed-on: https://gerrit.libreoffice.org/70876
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>