MDEV-20206 : Crash inside timer_callback()[threadpool_win.cc:419]

The most likely cause of the crash is that a timer fired, after it was closed.

MSDN documents such a possibility, in the documentation for
CloseThreadpoolTimer() function, and recommends disabling the timer before
calling WaitForThreadpoolTimerCallbacks()/CloseThreadpoolTimer().

The fix follows this recommendation.

Note, that 5.5-10.1 disabled the timer before close, but this code
was lost in threadpool refactoring in 10.2

sql/threadpool_win.cc

@@ -308,6 +308,7 @@ TP_connection_win::~TP_connection_win()
 
   if (timer)
   {
+    SetThreadpoolTimer(timer, 0, 0, 0);
     WaitForThreadpoolTimerCallbacks(timer, TRUE);
     CloseThreadpoolTimer(timer);
   }