ITS: Vertexer return early and only one arena

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

Author: f3sch

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

@@ -214,13 +214,16 @@ void VertexerTraits::computeTracklets(const int iteration)
           mTimeFrame->getNTrackletsROF(pivotRofId, 1) = std::accumulate(mTimeFrame->getNTrackletsCluster(pivotRofId, 1).begin(), mTimeFrame->getNTrackletsCluster(pivotRofId, 1).end(), 0);
         }
       });
-  });
 
-  mTimeFrame->computeTrackletsPerROFScans();
-  mTimeFrame->getTracklets()[0].resize(mTimeFrame->getTotalTrackletsTF(0));
-  mTimeFrame->getTracklets()[1].resize(mTimeFrame->getTotalTrackletsTF(1));
+    mTimeFrame->computeTrackletsPerROFScans();
+    if (auto tot0 = mTimeFrame->getTotalTrackletsTF(0), tot1 = mTimeFrame->getTotalTrackletsTF(1);
+        tot0 == 0 || tot1 == 0) {
+      return;
+    } else {
+      mTimeFrame->getTracklets()[0].resize(tot0);
+      mTimeFrame->getTracklets()[1].resize(tot1);
+    }
 
-  mTaskArena->execute([&] {
     tbb::parallel_for(
       tbb::blocked_range<short>(0, (short)mTimeFrame->getNrof()),
       [&](const tbb::blocked_range<short>& Rofs) {