Change ThreadPool shutdown to use deleteLater

This is to fix #9500, which has the frontend segfaulting on shutdown.  This
code will hopefully all be overhauled in the awaited webserver rewrite, but
in the mean time, let's have it behaving.

I also moved the order of removing from the deque and marking it for deletion,
as I was toying with different locking methods.

mythtv/libs/libmythupnp/threadpool.cpp

@@ -326,14 +326,11 @@ ThreadPool::~ThreadPool( )
     while (it != m_lstThreads.end() )
     {
         WorkerThread *pThread = *it;
-
-        if (pThread != NULL)
-            delete pThread;
-
         it = m_lstThreads.erase( it );
-    }
-
 
+        if (pThread != NULL) 
+            pThread->deleteLater();
+    }
 }
 
 /////////////////////////////////////////////////////////////////////////////