SAPT(DFT) might forget about freezing core

When one runs SAPT0 via the SAPT(DFT) pathway with the following options:
```
set SAPT_DFT_FUNCTIONAL HF
set SAPT_DFT_MP2_DISP_ALG FISAPT
energy('sapt(dft)')
```
the algorithm forgets to freeze the core in the exchange-dispersion term. This does not happen when `SAPT_DFT_MP2_DISP_ALG == 'FISAPT'`.

I'll submit a PR with the fix very soon.