fdo#70861: cui: fix crash in SvxExtParagraphTabPage

SvxExtParagraphTabPage::Reset() must not request a pool default for
SID_ATTR_PARA_PAGENUM because it simply does not exist; the item is
backed by a member of SwPageDesc in Writer.

(cherry picked from commit 9c1a1b98776ca2216d82af303394a09f2bac0471)
Signed-off-by: Andras Timar <andras.timar@collabora.com>

Conflicts:
	cui/source/tabpages/paragrph.cxx

Change-Id: I9b157f048cd986566f9b01f3677b53d5974b6c8d
(cherry picked from commit 7cbc78da69a755b8a994c46c5f04bee0b4b0dd1b)
Reviewed-on: https://gerrit.libreoffice.org/13176
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>