message: Eliminate some double-checking of Table validity
  _get_alloc()'s comment correctly notes that _get() is a place where it's safe
  to call as we check the Table before calling _get(), but _get() didn't actually
  use it.
diff: @@ -209,7 +209,7 @@ T Obj::_get(ColKey::Idx col_ndx) const
  {
  _update_if_needed();
  - typename ColumnTypeTraits<T>::cluster_leaf_type values(get_alloc());
  + typename ColumnTypeTraits<T>::cluster_leaf_type values(_get_alloc());
  ref_type ref = to_ref(Array::get(m_mem.get_addr(), col_ndx.val + 1));
  values.init_from_ref(ref);
  @@ -221,7 +221,7 @@ ObjKey Obj::_get<ObjKey>(ColKey::Idx col_ndx) const
  {
  _update_if_needed();
  - ArrayKey values(get_alloc());
  + ArrayKey values(_get_alloc());
  ref_type ref = to_ref(Array::get(m_mem.get_addr(), col_ndx.val + 1));
  values.init_from_ref(ref);
  