forcepoint#58 null deref

Change-Id: I826890ec85a16bc05fc1e4cd068079b0f8734d07
Reviewed-on: https://gerrit.libreoffice.org/58393
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

writerfilter/source/rtftok/rtfsdrimport.cxx

@@ -1119,6 +1119,8 @@ void RTFSdrImport::append(const OUString& aKey, const OUString& aValue)
 
 void RTFSdrImport::appendGroupProperty(const OUString& aKey, const OUString& aValue)
 {
+    if (m_aParents.empty())
+        return;
     uno::Reference<drawing::XShape> xShape(m_aParents.top(), uno::UNO_QUERY);
     if (xShape.is())
         applyProperty(xShape, aKey, aValue);