sw: paragraph-sign: validate current SwTextNode directly
When invoking undo, it turns out that the cursor position
is updated after the text modification, which triggers the
paragraph signature validation. Relying on the cursor
position, then, results in the wrong (previous) paragraph
to be validated (if the undo is in a different paragraph).
Since we have the correct SwTextNode when it's modified
(due to undo or otherwise), there is no reason why
we shouldn't use it and try to deduce it from the cursor.
Change-Id: I4c3283d59738988dcc1c592a9f3ef2c818ce675d
Reviewed-on: https://gerrit.libreoffice.org/63004
Tested-by: Jenkins
Reviewed-by: Ashod Nakashian <ashnakash@gmail.com>