Crash occurs with error "Missing files cannot have alt values"

In certain circumstances CHD files can be flagged with attributes that should not be possible. This causes RomVault to crash when attempting to fix.

Steps to reproduce:

1. Load a MAME listxml DAT (not Dir2Dat)

2. Scan a CHD that is needed for a fix in the MAME listxml DAT

3. Force RV to fail a Move operation by locking the CHD or assigning incompatible permissions

4. Find fixes / fix ROMs

5. RV will fail when attempting to move the CHD

6. The missing CHD will have an AltSHA1 assigned instead of SHA1, which should not be possible.

7. If you attempt to find and fix again RV will crash with an error "Missing files cannot have alt values"