ofz#860 clear old data before reading new data

Change-Id: I3bf5c2072a328052004c4c0551c2b125cb8ab19b
Reviewed-on: https://gerrit.libreoffice.org/35165
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>