Related rhbz#98977: Handle multiple LibXSLTTransformer::terminate calls

Trying to reproduce rhbz#98977, temporarily changing TRANSFORMATION_TIMEOUT_SEC
(filter/source/xsltfilter/XSLTFilter.cxx) to zero, loading a dummy "Microsoft
Excel 2003 XML (.xml)" file shows a "General input/output error" msg box and
"Cancel" leads to a crash due to duplicate calls to m_tcontrol->terminate() from
both XSLTFilter::importer and XSLTFilter::endDocument (both
filter/source/xsltfilter/XSLTFilter.cxx).

Change-Id: Ia103d944f3e1f14ca2cf5552ca3a48348132d38e

filter/source/xsltfilter/LibXSLTTransformer.cxx

@@ -463,9 +463,12 @@ namespace XSLT
 
     void
     LibXSLTTransformer::terminate() throw (RuntimeException)
+    {
+        if (m_Reader.is())
         {
             m_Reader->terminate();
             m_Reader->join();
+        }
         m_Reader.clear();
         m_parameters.clear();
     }