Related: fdo#71147 lwp-files open not or without charts/tables

It seems pContent may be NULL (see https://bugs.freedesktop.org/attachment.cgi?id=88539)

Change-Id: I80249dd1d0afc3feaef5f1b1d134210beee3d029
Reviewed-on: https://gerrit.libreoffice.org/6536
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>
(cherry picked from commit a210df8e50d94f557ceaa21e5f4943db97777812)