Fix FillPatcher after enforcing PhysBCFunct operator() fills nghost

[cgilet]

grow cells (PR 3914)

Also assert in operator that mf has nghost grow cells to fill.

Summary

Additional background

Checklist

The proposed changes:

• [ ] fix a bug or incorrect behavior in AMReX
• [ ] add new capabilities to AMReX
• [ ] changes answers in the test suite to more than roundoff level
• [ ] are likely to significantly affect the results of downstream AMReX users
• [ ] include documentation in the code and/or rst files, if appropriate

[WeiqunZhang]

We also need to change Line 596.