Security: don't pass a string to fgValidatePath then use the original

Security: don't pass a string to fgValidatePath then use the original

This is insecure because it always (not just on Windows) converts
\ to / before .. checking.  Either use the path it returns (as in
f_open()) or use an SGPath (where this conversion is already done)

Only a minor problem because the affected functions are limited to
the .sav file type