dtls: fix faulty offsets and tvbuff references

This could lead to problems for fragmented DTLS packets.

Change-Id: I602c7e181ea3799a4a2e7bcfed05bfbb129f7df4
Reviewed-on: https://code.wireshark.org/review/3017
Reviewed-by: Peter Wu <peter@lekensteyn.nl>
Reviewed-by: Evan Huus <eapache@gmail.com>