Fix xundump's "value must match" check for split tables
The check applies to selectors with flag NSC_CONST set. It permits
initializing them in new objects, but prevents changing them in
existing objects. For split tables, initialization worked only in the
first part, because new objects were considered old in later parts.
For instance, in a custom config sect-chr with mnem in the second
part's field 2, new sector types were rejected with `Value for field 2
must be ""'.