Lua: Only register postdissectors once.

Change-Id: Id41e4ebd19e0b80faa06f628d689bdb77f2b84c9
Reviewed-on: https://code.wireshark.org/review/5023
Reviewed-by: Stig Bjørlykke <stig@bjorlykke.org>
(cherry picked from commit c6feecff74a50d9c87c71ca937d831e0c345271f)
Reviewed-on: https://code.wireshark.org/review/5024