SNA: Fix Dead Store (Dead assignement/Dead increment) warning found by Clang

Change-Id: I7a491488e67890c6cde98beec6a4935b10505afb
Reviewed-on: https://code.wireshark.org/review/5555
Petri-Dish: Alexis La Goutte <alexis.lagoutte@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Anders Broman <a.broman58@gmail.com>