If SwCache::Insert() happens to delete the object that m_pFirst or
m_pRealFirst point to, which is unlikely as it means every other object
is locked, then these pointers must be updated.

This sometimes happens in the bugdoc after scrolling around for some
time.

Change-Id: I13f04d28c37969469efa4e1109c7f5b751ceba96
Reviewed-on: https://gerrit.libreoffice.org/49151Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Michael Stahl <mstahl@redhat.com>
(cherry picked from commit 269d6d33)
Reviewed-on: https://gerrit.libreoffice.org/49234Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>