IPv6 RPL option is read as less bytes than it is

Add check if the opt len equal to the "calculcate" length
and display expert info if not.. also replace offset by the opt len

Bug:10559
Bug:10561
Change-Id: I6e80e4055b6d0c5ef3f030c544cd1629e6b9abec
Reviewed-on: https://code.wireshark.org/review/4627
Reviewed-by: Evan Huus <eapache@gmail.com>