Use a single exit_status instead of two

src/threads/thread.c

@@ -673,7 +673,7 @@ init_process_result (struct thread *t)
 {
   struct process_result *result = malloc (sizeof (struct process_result));
   result->tid = t->tid;
-  result->exit_status = t->exit_status;
+  result->exit_status = -1;
   lock_init (&result->lock);
   sema_init (&result->sema, 0);
   t->result = result;
@@ -707,8 +707,6 @@ init_thread (struct thread *t, const char *name, int nice, int priority,
   t->recent_cpu = recent_cpu;
   t->priority = t->base_priority;
 
-  t->exit_status = -1;
-
   old_level = intr_disable ();
   list_push_back (&all_list, &t->allelem);
   intr_set_level (old_level);