fdo#35712 Correct update of equation in status bar

Change-Id: I6db5628bbad337f70e3a39a4c65afbd0f6c30a9d
Reviewed-on: https://gerrit.libreoffice.org/6658
Tested-by: Tomaž Vajngerl <quikee@gmail.com>
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>