fix floating table over section with several columns (part of bnc#875383)

Change-Id: Ic6e75cc2cedb61754b45bc4678a1185f580d5ed6
Reviewed-on: https://gerrit.libreoffice.org/10431
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>