Lua: Fix for register dissectors only once.

The postdissector may be registered before adding the dissector function.

Change-Id: I2285824835491ac91e00515bfb18eb471888eaf8
Reviewed-on: https://code.wireshark.org/review/5106
Petri-Dish: Stig Bjørlykke <stig@bjorlykke.org>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Stig Bjørlykke <stig@bjorlykke.org>
Tested-by: Stig Bjørlykke <stig@bjorlykke.org>