rhbz#1065629: RTF import: don't drop nested cells if not enough \cellx

In this document written by "XMLmind XSL-FO Converter" there are less
\cellx than \cell and thus when reading \nestrow/\row a whole buffered
nested table \cell is lost and then subsequently the rest of the nested
table too.  Try to fix that by counting both \cell and \cellx and
replaying until the maximum of those.

Cannot count \intbl since we synthesize that in various places.

(regression in LO 3.5)

Change-Id: I3b64ad94af842e076611418589a0c83bd18841c6
(cherry picked from commit 07ef4cf096015f0e427ffd17cd26bb6837e75481)
Reviewed-on: https://gerrit.libreoffice.org/8100
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>