wireshark: detect and skip obviously broken messages

When a TCP segment is lost Wireshark may attempt to start a PDU in the middle
of an OpenFlow message. This breaks framing and can make the rest of the
capture useless. These checks attempt to detect this and skip this segment,
hopefully restarting dissection at the beginning of the next message.
wireshark: use desegment_offset

If the second (or later message) in a segment needs to be reassembled from
further segments, it's not enough to tell wireshark how long the message is. We
also need to tell it what the offset is from the start of the current segment.
Merge into master from pull request #279:
wireshark: improve dissection in case of missing segments (https://github.com/floodlight/loxigen/pull/279)