ITS: fix spurious error accessing empty tracklets

Signed-off-by: Felix Schlepper <felix.schlepper@cern.ch>

Author: f3sch

Detectors/ITSMFT/ITS/tracking/src/VertexerTraits.cxx

@@ -269,18 +269,20 @@ void VertexerTraits::computeTracklets(const int iteration)
   if (mTimeFrame->hasMCinformation()) {
     for (const auto& trk : mTimeFrame->getTracklets()[0]) {
       o2::MCCompLabel label;
-      int sortedId0{mTimeFrame->getSortedIndex(trk.rof[0], 0, trk.firstClusterIndex)};
-      int sortedId1{mTimeFrame->getSortedIndex(trk.rof[1], 1, trk.secondClusterIndex)};
-      for (const auto& lab0 : mTimeFrame->getClusterLabels(0, mTimeFrame->getClusters()[0][sortedId0].clusterId)) {
-        for (const auto& lab1 : mTimeFrame->getClusterLabels(1, mTimeFrame->getClusters()[1][sortedId1].clusterId)) {
-          if (lab0 == lab1 && lab0.isValid()) {
-            label = lab0;
+      if (!trk.isEmpty()) {
+        int sortedId0{mTimeFrame->getSortedIndex(trk.rof[0], 0, trk.firstClusterIndex)};
+        int sortedId1{mTimeFrame->getSortedIndex(trk.rof[1], 1, trk.secondClusterIndex)};
+        for (const auto& lab0 : mTimeFrame->getClusterLabels(0, mTimeFrame->getClusters()[0][sortedId0].clusterId)) {
+          for (const auto& lab1 : mTimeFrame->getClusterLabels(1, mTimeFrame->getClusters()[1][sortedId1].clusterId)) {
+            if (lab0 == lab1 && lab0.isValid()) {
+              label = lab0;
+              break;
+            }
+          }
+          if (label.isValid()) {
             break;
           }
         }
-        if (label.isValid()) {
-          break;
-        }
       }
       mTimeFrame->getTrackletsLabel(0).emplace_back(label);
     }