message: Fix crash in 1qlz PDB file from 2nd Structure Assignment
  Check to make sure atoms are valid before using them
diff: @@ -181,6 +181,7 @@ void SecondaryStructureAssigner::assignBackboneHydrogenBonds()
  continue;
  auto oxygen = residue.getAtomByName("O");
  + if (oxygen.isValid()) {
  hBondRecord* oRecord = new hBondRecord();
  oRecord->atom = oxygen.index();
  oRecord->atomZ = oxygen.position3d()[2];
  @@ -188,8 +189,10 @@ void SecondaryStructureAssigner::assignBackboneHydrogenBonds()
  oRecord->residue = residueId;
  oRecord->residuePair = residueId; // just a placeholder
  m_hBonds.push_back(oRecord);
  + }
  auto nitrogen = residue.getAtomByName("N");
  + if (nitrogen.isValid()) {
  hBondRecord* nRecord = new hBondRecord();
  nRecord->atom = nitrogen.index();
  nRecord->atomZ = nitrogen.position3d()[2];
  @@ -198,6 +201,10 @@ void SecondaryStructureAssigner::assignBackboneHydrogenBonds()
  nRecord->residuePair = residueId;
  m_hBonds.push_back(nRecord);
  }
  + }
  +
  + if (m_hBonds.size() == 0)
  + return;
  // sort by z-coordinate
  std::sort(m_hBonds.begin(), m_hBonds.end(),
  