TLS: Don't assert on zero length handshake fragment

The fragment functions will work with a zero length fragment, which might happen if a record length is zero in a malformed packet and a reassembly is in progress. It is not by itself a fatal error (and could actually work, even though non-compliant.) There is already a tls.record.length.invalid expert info added by ssl_check_record_length for this case.

Related to #17890.