MDEV-5913 Windows: 10.0 crashes on shutdown

add pthread_mutex_destroy(&THR_LOCK_dbug); that was apparently
forgotten since the very first MySQL version

dbug/dbug.c

@@ -1642,6 +1642,7 @@ void _db_end_()
 
   cs->stack= &init_settings;
   FreeState(cs, 0);
+  pthread_mutex_destroy(&THR_LOCK_dbug);
   init_done= 0;
 }