do not store invalid date acceptance patterns in configuration

(cherry picked from commit e0480c81a956751e48f8ef36a41c3062c1bed345)

Conflicts:
	cui/source/options/optgdlg.cxx

Change-Id: I78cd8b304db2243fd105d4b13421b6ea0347e042
Reviewed-on: https://gerrit.libreoffice.org/9453
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>