NTP: fix parsing of control assignments with empty value

Consider that ',' is a separator between assignments and make value presence optional

Bug: 10417
Change-Id: I23f2b4029548a1263d65ee11c6356270e7a89bd7
Reviewed-on: https://code.wireshark.org/review/3884
Reviewed-by: Evan Huus <eapache@gmail.com>
Petri-Dish: Alexis La Goutte <alexis.lagoutte@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>
(cherry picked from commit edcd2dd57f1649dbb25f1ad0f4a3a77c4d454322)
Reviewed-on: https://code.wireshark.org/review/3889
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>