Fix callback return inside of promise causing undefined.

Author: sjudson

lib/key/private.js

@@ -194,7 +194,7 @@ function _public(cb) {
         ? extcrypto.extract(self.raw())
         : Buffer.from(sodium.crypto_sign_ed25519_sk_to_pk(self.raw()));
 
-  return pk.inject(rkey, (err) => {
+  return pk.inject(rkey).then((err) => {
     if (err) { return done(err); }
     return done(null, pk);
   });