sw: avoid divide by zero in SwTextGridPage::CharorLineChangedHdl
See https://crashreport.libreoffice.org/stats/signature/SwTextGridPage::CharorLineChangedHdl(weld::SpinButton%20&)
disable the offending widget if the value is zero.
crash reproducible under gen with CJK features enabled by: format, page
style, text grid, grid (lines and characters), max base text size of
1000 and then click in "characters per line" and then click in "lines
per page".
Change-Id: Ic0f8131955ab01412dd54bdd5d3bffe518938fb5
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/136470
Tested-by: Jenkins
Reviewed-by: Xisco Fauli <xiscofauli@libreoffice.org>