Resolves: fdo#53460 crashes after casting SwTxtFrm to unrelated SwLayoutFrm

We still can't use ctrl+up to shrink the height of a row that has a table in
it, but it doesn't crash anymore.

This code presumably isn't truly table-in-table aware and should somehow step
"over" the embedded table and not "into" it, which is what I guess it is doing
here.

(cherry picked from commit 5141f2e0d89a8a10f0009bea40cc5cd15bf4fcc8)

Conflicts:
	sw/source/core/layout/frmtool.cxx

Change-Id: I0e4c757c75438a89eb7721de32990f2f21c1ad8b
(cherry picked from commit c1a5e49bdb0d6c1533d33520ea7c01a660a5d1f4)
Reviewed-on: https://gerrit.libreoffice.org/13412
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>