SfxMedium::GetOutputStream(): re-use existing XStream

The ScExportTest::testConditionalFormatExportXLSX() fails on Windows
because of how SfxMedium handles its streams:
1. SfxMedium::GetOutputStorage() creates some temp file
2. SfxMedium::GetMedium_Impl() opens a XStream on the temp file
3. SfxMedium::GetOutStream() wants to open a SvFileStream on the temp
   file, but because the file is already open and the sharing options
   are set to deny sharing, opening fails with ERROR_SHARING_VIOLATION

Prevent that by re-using the already open XStream in GetOutStream.
Hopefully this does not break anything, and there is already a comment
in CloseInStream_Impl() indicating that m_pOutStream and xStream are
related.

(interestingly ERROR_SHARING_VIOLATION is documented to occur if
_another_ process has the file open, but evidently it happens here on
NT 6.1 for the same process...)

Change-Id: I6d2ec36fd45a0317e947ddfb436472a8b86fbe26

sfx2/source/doc/docfile.cxx

@@ -637,7 +637,20 @@ SvStream* SfxMedium::GetOutStream()
 
         if ( pImp->pTempFile )
         {
-            pImp->m_pOutStream = new SvFileStream( pImp->m_aName, STREAM_STD_READWRITE );
+            // try to re-use XOutStream from xStream if that exists;
+            // opening new SvFileStream in this situation may fail on
+            // Windows with ERROR_SHARING_VIOLATION
+            if (pImp->xStream.is())
+            {
+                assert(pImp->xStream->getOutputStream().is()); // need that...
+                pImp->m_pOutStream = utl::UcbStreamHelper::CreateStream(
+                        pImp->xStream, false);
+            }
+            else
+            {
+                pImp->m_pOutStream = new SvFileStream(
+                        pImp->m_aName, STREAM_STD_READWRITE);
+            }
             CloseStorage();
         }
     }