message: fix null ptr
diff: @@ -77,7 +77,7 @@ void AliAodSkimTask::CleanTrack(AliAODTrack *t)
  AliAODPid *pid = t->GetDetPid();
  delete pid;
  t->SetDetPID(0);
  - } else {
  + } else if (t->GetDetPid()) {
  AliAODPid *pid = t->GetDetPid();
  AliAODPid *nid = new AliAODPid;
  nid->SetTPCsignal(pid->GetTPCsignal());
  