Reset missing ROBs vector before calling eventCache_checkRobListToCache

Patch sent by @wiedenma

HLT/Trigger/TrigControl/TrigServices/src/HltROBDataProviderSvc.cxx

@@ -387,6 +387,7 @@ void HltROBDataProviderSvc::getROBData(const EventContext& context,
 
   // return all the requested ROB fragments from the cache
   robFragments.clear() ;
+  robIds_missing.clear() ;
   eventCache_checkRobListToCache(cache, robIds, robFragments, robIds_missing) ;
 }