/*
 * Finalize generated signature.
 *
 * Call this after MusicBrainz::TRM#generate_signature has returned true.
 *
 * 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) {