fight the symptoms of BZDNG-154

Not a complete fix: Accessing AtomHandles, ResidueHandles and ChainHandles whose
EntityHandle is out of scope still crashes. That requires larger refactoring
though.

modules/mol/base/src/atom_view.cc

@@ -67,7 +67,10 @@ AtomView::AtomView(const ResidueView& residue_view,
 ResidueView AtomView::GetResidue() const 
 {
   this->CheckValidity();
-  return ResidueView(data_->residue.lock(), Impl()->GetResidue());
+  if (!data_->residue.expired()) {
+    return ResidueView(data_->residue.lock(), Impl()->GetResidue());    
+  }
+  throw InvalidHandle();
 }
 
 void AtomView::Apply(EntityVisitor& visitor) 

modules/mol/base/src/chain_view.cc

@@ -79,7 +79,11 @@ ChainView::ChainView() {
 }
 
 EntityView ChainView::GetEntity() const {
-  return EntityView(data_->entity.lock(), Impl()->GetEntity());
+  this->CheckValidity();
+  if (!data_->entity.expired()) {
+    return EntityView(data_->entity.lock(), Impl()->GetEntity());    
+  }
+  throw InvalidHandle();
 }
 
 ChainView::ChainView(const EntityView& entity,

modules/mol/base/src/residue_view.cc

@@ -138,7 +138,10 @@ const AtomViewList& ResidueView::GetAtomList() const {
 
 ChainView ResidueView::GetChain() const {
   this->CheckValidity();
-  return ChainView(data_->chain.lock(), Impl()->GetChain());
+  if (!data_->chain.expired()) {
+    return ChainView(data_->chain.lock(), Impl()->GetChain());
+  }
+  throw InvalidHandle();
 }