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>
(cherry picked from commit 9e5e43ba449b4431f9db0cb77cd0fa3171acf7d4)
Reviewed-on: https://code.wireshark.org/review/3028
Reviewed-by: Hauke Mehrtens <hauke@hauke-m.de>
Reviewed-by: Bill Meier <wmeier@newsguy.com>