Join the thread only if launch succeeded.

Change-Id: Idee779cea587e11f6d0f7902182c9394e73d46eb
Reviewed-on: https://gerrit.libreoffice.org/60488
Tested-by: Jenkins
Reviewed-by: Michael Meeks <michael.meeks@collabora.com>

sax/source/fastparser/fastparser.cxx

@@ -814,8 +814,8 @@ void FastSaxParserImpl::parseStream(const InputSource& rStructSource)
     {
         rtl::Reference<ParserThread> xParser;
         xParser = new ParserThread(this);
-        aEnsureFree.setThread(xParser);
         xParser->launch();
+        aEnsureFree.setThread(xParser);
         bool done = false;
         do {
             rEntity.maConsumeResume.wait();