Lua: Register dissectors only once.

Change-Id: I883e8560e9d101d34760d09715255e4fccc9eb98
Reviewed-on: https://code.wireshark.org/review/5022
Reviewed-by: Hadriel Kaplan <hadrielk@yahoo.com>
Reviewed-by: Stig Bjørlykke <stig@bjorlykke.org>
Tested-by: Stig Bjørlykke <stig@bjorlykke.org>