[lmi] _M_range_check fails when running "new" PDF

[Greg Chicares]

We see this message:
  vector::_M_range_check: __n (which is 0) >= this->size() (which is 0)
with ledger type mce_individual_private_placement when we run an
"inforce" illustration. "Inforce" means, e.g., that you bought a
life insurance policy in the past, and now you want an illustration
that starts today (as opposed to starting when the policy was sold).

To reproduce:

  $wine ./lmi_wx_shared --ash_nazg --data_path=/opt/lmi/data --pyx=only_new_pdf

[it's also reproducible without "--pyx=only_new_pdf"]

  File | Open | "sample.database"  [in data directory]
    Miscellanea ... LedgerType: change value to "6" (digit six)
  File | Save

  File | New | Illustration
    "Effective date": select year, press down-arrow once
    [today, e.g., that changes "2018-02-13" to "2017-02-13"]
  OK
  File | Print to PDF

Looks like an at() call is failing.