Update thread_set_priority to not do anything if priority is unchanged

src/threads/thread.c

@@ -383,6 +383,10 @@ thread_set_priority (int new_priority)
   ASSERT (new_priority >= PRI_MIN);
   ASSERT (new_priority <= PRI_MAX);
 
+  int old_priority = thread_get_priority ();
+  if (new_priority == old_priority)
+    return;
+
   thread_current ()->priority = new_priority;
 }