Added function dnp3_header_check() to check the validity of a DNP3
packet as far as possible, called from both dissect_dnp3_tcp and dissect_dnp3_udp.

Bug: 10287
Change-Id: Iaa988258b3614cb1b408dec41a987fbd61c9727c
Reviewed-on: https://code.wireshark.org/review/3096
Petri-Dish: Graham Bloice <graham.bloice@trihedral.com>
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>
Reviewed-by: Graham Bloice <graham.bloice@trihedral.com>