ofproto: Only dump queue statistics if the queue really exists.

ofproto: Only dump queue statistics if the queue really exists.

Without this commit, "ovs-ofctl queue-stats br0 ALL 1" will print something
like the following if port 3 has queue 1 but none of the other ports do:

    stats_reply (xid=0x7b378): flags=none type=5(queue)
     4 queues
      port 0 queue 1: bytes=?, pkts=?, errors=?
      port 1 queue 1: bytes=?, pkts=?, errors=?
      port 2 queue 1: bytes=?, pkts=?, errors=?
      port 3 queue 1: bytes=0, pkts=0, errors=0

With this commit, it will print the following instead, which seems more
useful:

   stats_reply (xid=0x3ada1): flags=none type=5(queue)
    1 queues
     port 3 queue 1: bytes=0, pkts=0, errors=0