Fix bug in OnlineMessageSvc that causes abort in debug builds with Gaudi v29r3.

CommonMessaging changed in Gaudi v29r3, which results in an abort with an uncaught exception when the OnlineMessageSvc calls Service::initialize. This MR fixes that problem.

The production stack for 2018 will be based on Gaudi v29r3, so at the time the version of Gaudi used by Online is updated, this MR can be merged.