Check hopcount is correctly incremented by 1

Extract RFC3315 about hop-count :
20.1.2. Relaying a Message from a Relay Agent

   If the message received by the relay agent is a Relay-forward message
   and the hop-count in the message is greater than or equal to
   HOP_COUNT_LIMIT, the relay agent discards the received message.

   The relay agent copies the source address from the IP datagram in
   which the message was received from the client into the peer-address
   field in the Relay-forward message and sets the hop-count field to
   the value of the hop-count field in the received message incremented
   by 1.

Bug:10449
Change-Id: Ifb94e7c54c0a26714fc543862d4358d3e60c2676
Reviewed-on: https://code.wireshark.org/review/4017
Reviewed-by: Michael Mann <mmann78@netscape.net>
Petri-Dish: Michael Mann <mmann78@netscape.net>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Evan Huus <eapache@gmail.com>