[ns] A ns-2.1b8a bug in PacketQueue::getnext()

["Zhou, Zhi Qiang (Roger)" <zroger@lucent.com>]

Orignal:

	 Packet* PacketQueue::getNext() { 
		Packet *tmp = iter; iter = iter->next_;
		return tmp;
	}

Should be:
	 Packet* PacketQueue::getNext() { 
		if (!iter) return NULL;
		Packet *tmp = iter; iter = iter->next_;
		return tmp;
	}


best regards,

zroger