Don't crash when waiting for thread termination in wxMSW.

The changes of r65882 adding wxThread::On{Delete,Kill}() introduced a bug
which made normal wxThread::Wait() crash.

Fix it by calling OnDelete() only if we're really deleting a thread.

Closed #12627, see #9046.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65939 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/msw/thread.cpp

@@ -761,7 +761,8 @@ wxThreadInternal::WaitForTerminate(wxCriticalSection& cs,
         Cancel();
     }
 
-    threadToDelete->OnDelete();
+    if ( threadToDelete )
+        threadToDelete->OnDelete();
 
     // now wait for thread to finish
     if ( wxThread::IsMain() )