[DPG] Skip ambiguous tracks with an empty BC slice in the event selection QA (#18019)

Author: sawenzel

DPG/Tasks/AOTEvent/eventSelectionQa.cxx

@@ -1009,8 +1009,12 @@ struct EventSelectionQaTask {
 
       // special check to avoid crashes (in particular, on some MC Pb-Pb datasets)
       // (related to shifts in ambiguous tracks association to bc slices (off by 1) - see https://mattermost.web.cern.ch/alice/pl/g9yaaf3tn3g4pgn7c1yex9copy
-      if (ambTrId >= 0 && (ambTracks.iteratorAt(ambTrId).bcIds()[0] >= bcs.size()))
-        continue;
+      // an empty slice (-1, -1) passes the upper check, so test it as well
+      if (ambTrId >= 0) {
+        const auto& ambTrack = ambTracks.iteratorAt(ambTrId);
+        if (!ambTrack.has_bc() || ambTrack.bcIds()[0] >= bcs.size() || ambTrack.bcIds()[1] >= bcs.size())
+          continue;
+      }
 
       int indexBc = ambTrId < 0 ? track.collision_as<ColEvSels>().bc_as<BCsRun3>().globalIndex() : ambTracks.iteratorAt(ambTrId).bc_as<BCsRun3>().begin().globalIndex();
       auto bc = bcs.iteratorAt(indexBc);