update main thread "properly" in headless case

Change-Id: Ie76555a007db13e89f06c85d02beebf84eadea0b
Reviewed-on: https://gerrit.libreoffice.org/62912Reviewed-by: Michael Meeks <michael.meeks@collabora.com>
Tested-by: Michael Meeks <michael.meeks@collabora.com>

vcl/headless/svpinst.cxx

@@ -390,7 +390,10 @@ bool SvpSalInstance::IsMainThread() const
 void SvpSalInstance::updateMainThread()
 {
     if (!IsMainThread())
+    {
         m_MainThread = osl::Thread::getCurrentIdentifier();
+        ImplGetSVData()->mnMainThreadId = osl::Thread::getCurrentIdentifier();
+    }
 }
 
 bool SvpSalInstance::DoYield(bool bWait, bool bHandleAllCurrentEvents)