fix crash loading ooo#93570-3.doc

regression from f2945255df273404ee2457dcf761cb8f334b732b
cp#2013101510000026: doc import of comments affecting more text nodes

use Move(fnMoveBackward, fnGoNode) to at least ensure we stop going
backwards when we run out of valid places to go backwards to.

This still isn't great because the distance between two msword character
indexes only equates to the same distance between our characters in the very
simple cases

Change-Id: I248fd12c067577d2f1fd64f48583321eb6d453e4
(cherry picked from commit b1cd83c625a2afeb9da43cc9745d79c01963c797)
Reviewed-on: https://gerrit.libreoffice.org/10099
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>