artnet: fix crash
artnet: fix crash

happens when the engine thread calls writeUniverse
while the UI (Qt) thread is in handlePacket

sbenejam/qlcplus,plugz/qlcplus,kripton/qlcplus,plugz/qlcplus,mcallegari/qlcplus,plugz/qlcplus,sbenejam/qlcplus,plugz/qlcplus,plugz/qlcplus,nedmech/qlcplus,kripton/qlcplus,nedmech/qlcplus,kripton/qlcplus,sbenejam/qlcplus,plugz/qlcplus,plugz/qlcplus,mcallegari/qlcplus,kripton/qlcplus,kripton/qlcplus,sbenejam/qlcplus,nedmech/qlcplus,nedmech/qlcplus,sbenejam/qlcplus,kripton/qlcplus,sbenejam/qlcplus,nedmech/qlcplus,mcallegari/qlcplus,mcallegari/qlcplus,mcallegari/qlcplus,mcallegari/qlcplus,mcallegari/qlcplus,sbenejam/qlcplus,nedmech/qlcplus,kripton/qlcplus