Adjust for encoding in fdata

[achaikou]

Encoding is supported for EFLRs, but not always for IFLRs, currently we receive error 'could not allocate string object' when utf-8 is broken. In actual files issue happens by accident when actual fdata is broken.
Existing test test_fdata/test_broken_ut8_ascii.

[ErlendHaa]

This can be cased by either 1) non-utf8 ident in the fdata obname or 2) if records is somehow truncated. 1 is solved as fdata now returns dl::ident, not std::string, while 2 is a particular case of #295