Added protection on event slot creation in HiveWhiteBoard

So far the code could try to create a new event even if no slots were available. This was due to the fact that there was not count of free slots. The consequences where an exception out of range further down the road as the return code of the event creation was on top not checked properly. This adds a free slot counter in the white board

GaudiHive/src/HiveSlimEventLoopMgr.cpp

@@ -456,7 +456,8 @@ StatusCode HiveSlimEventLoopMgr::nextEvent( int maxevt )
     if ( ( newEvtAllowed or createdEvts == 0 ) &&  // Launch the first event alone
          createdEvts >= 0 &&                       // The events are not finished with an unlimited number of events
          ( createdEvts < maxevt or maxevt < 0 ) && // The events are not finished with a limited number of events
-         m_schedulerSvc->freeSlots() > 0 ) {       // There are still free slots in the scheduler
+         m_schedulerSvc->freeSlots() > 0 &&        // There are still free slots in the scheduler
+         m_whiteboard->freeSlots() > 0 ) {         // There are still free slots in the whiteboard
 
       if ( 1 == createdEvts ) // reset counter to count from event 1
         start_time = Clock::now();