lists.c

// Function
static void clean_references(struct MuttThread *brk, struct MuttThread *cur);

// Original Code
for (ref = cur->message->env->references; p->message && ref; ref = ref->next)
  if (mutt_strcasecmp(ref->data, p->message->env->message_id) == 0)
  {
    done = true;
    break;
  }

// New Code
for (ref = STAILQ_FIRST(&cur->message->env->references);
     p->message && ref;
     ref = STAILQ_NEXT(ref, entries))
{
  if (mutt_strcasecmp(ref->data, p->message->env->message_id) == 0)
  {
    done = true;
    break;
  }
}

// Suggestion
STAILQ_FOREACH(ref, &cur->message->env->references, entries)
{
  if (!p->message)
    break;
  if (mutt_strcasecmp(ref->data, p->message->env->message_id) == 0)
  {
    done = true;
    break;
  }
}