lbtrm: Use correct variable for outgoing_mc_address.

Change-Id: I17e06e39de0de347926825741897646507691657
Reviewed-on: https://code.wireshark.org/review/5155
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>
(cherry picked from commit b479ab4172b3dc8b4678a62d1c0611a2b1827ab8)
Reviewed-on: https://code.wireshark.org/review/5159