Fix (unreported) Broken `BLI_threadapi_exit()`.

Function would not clear the static scheduler pointer, which lead to
crash (mem use after free) when trying to re-init and use the task API
again. Should not happen in Blender itself, but could in other cases
(like some future gtests ;) ).

source/blender/blenlib/intern/threads.c

@@ -161,6 +161,7 @@ void BLI_threadapi_exit(void)
 {
   if (task_scheduler) {
     BLI_task_scheduler_free(task_scheduler);
+    task_scheduler = NULL;
   }
   BLI_spin_end(&_malloc_lock);
 }