Packet out message with no data causes crash

[KitL]

Hi,

I hit a problem with packet out messages when they are used to output buffered packets. When you do this you send a packet out message with a buffer_id != -1 and no data. But you still try to unpack the empty data field.

Traceback (most recent call last):
  File "ofp_sniffer.py", line 234, in <module>
    main()
  File "ofp_sniffer.py", line 222, in main
    sniffer.run()
  File "ofp_sniffer.py", line 90, in run
    self.cap.loop(-1, self.process_packet)
  File "ofp_sniffer.py", line 161, in process_packet
    self.ofp_proxy.process_packet(pkt)
  File "/opt/ofp_sniffer/apps/ofp_proxies.py", line 104, in process_packet
    lldp = get_protocol(msg.ofp.data, lldp=True)
  File "/opt/ofp_sniffer/libs/tcpiplib/process_data.py", line 165, in get_protocol
    data = dissect_data(data)
  File "/opt/ofp_sniffer/libs/tcpiplib/process_data.py", line 26, in dissect_data
    eth.parse(packet[start:start + 14], 1)
  File "/opt/ofp_sniffer/libs/tcpiplib/packet.py", line 73, in parse
    ethernet = unpack('!6s6sH', eth_raw)
struct.error: unpack requires a buffer of 14 bytes

The length of eth_raw it seems is supposed to be enforced by slicing data---in line 26 in dissect_data, and in line 72 of parse---but this doesnt seem to actually work, the length is still 0:

>>> a = bytes('', 'utf-8')
>>> len(a)
0
>>> b = a[0:14]
>>> len(b)
0

I put together a really rudimentary patch, but it looks as though there are a few places this assumption is being made, it isnt crashing now but I get a lot of Error: unpack requires a buffer of 14 bytes popping up in my output. I figure it is probably better for me to hand this over to someone with a better idea of when and where this code is hit.