Fix a crash by preventing removal of mandatory footprint fields

When certain layers of the PCB are disabled, copying and pasting a footprint that has items on the disabled layers may cause the new footprint's mandatory fields to be pruned. Attempting to copy and paste this copy again, KiCad will crash.

Create a new PCB and place a footprint:

Disable F.Fab, copy this footprint and paste, the new footprint will lost mandatory fields:

Try copy the new footprint, crash.