disk.cpp: misc changes

- added a "missing" + case ADF_SCP: break; originating from the scp support patch from Keith. **please double-check if this should be included**
- Misc minor compilation warning fixes
- added a check (currprefs.floppyslots[dnum].df != fname) before copying strings, in case they already point to the same address

This shouldn't be moved, it should still be seen by debugger and other parts of emulator even if result isn't stored.