forcepoint #20 pop on empty stack

Thanks to Antti Levomäki and Christian Jalio from Forcepoint.

Change-Id: I3098c5d32d8f97dff4c620e054acde1ba1b6adf3
Reviewed-on: https://gerrit.libreoffice.org/50972Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

writerfilter/source/rtftok/rtfsdrimport.cxx

@@ -132,7 +132,8 @@ void RTFSdrImport::popParent()
 {
     if (!m_aGraphicZOrderHelpers.empty())
         m_aGraphicZOrderHelpers.pop();
-    m_aParents.pop();
+    if (!m_aParents.empty())
+        m_aParents.pop();
 }
 
 void RTFSdrImport::resolveDhgt(uno::Reference<beans::XPropertySet> const& xPropertySet,