/*
* Finalize generated signature.
*
* Call this after MusicBrainz::TRM#generate_signature has returned 1.
*
* Accepts an optional 16-byte string, used to associate the signature
* with a particular collection in the RElatable Engine. Returns nil on
* error, or a 16 byte signature on success.
*
* Example:
* sig = trm.finalize_signature
*
*/
static VALUE mb_trm_finalize_sig(int argc, VALUE *argv, VALUE self) {