fdo#81173 abi#3623 RTF import: missing null pointer check

(cherry picked from commit 08fb25ab7941c52d4eed0435a45a38b03aff588c)
Signed-off-by: Michael Stahl <mstahl@redhat.com>

Change-Id: I5861e5fec0d822f42dfeb994500a818a968e69fb
(cherry picked from commit 50df6263ccde24874191812950dcccd8f839754b)
Reviewed-on: https://gerrit.libreoffice.org/10358
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>
Reviewed-by: Andras Timar <andras.timar@collabora.com>