In Visual Studio builds, enable the L4 warning
4189: Local variable is initialized but not referenced

Fix some variables found by this

Change-Id: Icc13def5413f1fe885ec25e659462c8906a6cfa0
Reviewed-on: https://code.wireshark.org/review/3748
Reviewed-by: Graham Bloice <graham.bloice@trihedral.com>
Petri-Dish: Graham Bloice <graham.bloice@trihedral.com>
Petri-Dish: Alexis La Goutte <alexis.lagoutte@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Evan Huus <eapache@gmail.com>