6LoWPAN: fix encapsulating address decoding when a Mesh header is present

Bug: 10462
Change-Id: I37d532edc41193123b1a7bb380d48cb76c118bf4
Reviewed-on: https://code.wireshark.org/review/4161
Petri-Dish: Pascal Quantin <pascal.quantin@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>
Reviewed-by: Michael Mann <mmann78@netscape.net>
(cherry picked from commit 1e47e050dcdf30eced478e6909408d79548975c6)
Reviewed-on: https://code.wireshark.org/review/4227