Skip ambiguous tracks with an empty BC slice in the event selection QA

This fixes a crash of the event selection QA task on MC AODs.

- An ambiguous track can carry the BC slice (-1, -1), which means it has no
  associated BC at all.
- The existing guard only rejected slices at or beyond the end of the BC table,
  so (-1, -1) passed it and bc_as().begin().globalIndex() was called on an empty
  slice, which segfaults.
- The guard now uses has_bc(), which tests both slice ends for being negative,
  as CollisionAssociation.h already does, and checks the upper end as well.
- Reproduced on LHC26a5a_gp_2025_v10 with O2Physics daily-20260729-0000-1, where
  the task exits 128; with this change the same file is processed and the QA
  histograms are filled.

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);