return null on error with signed objects

test/index.js

@@ -107,6 +107,13 @@ test ('verify_signature', (t) => {
   t.deepEqual (obj, dec);
 });
 
+test ('return null on invalid input', (t) => {
+  const ver = crypto.verify_signature (null, 'foo');
+  t.is (ver, null);
+  const dec = crypto.decode_signed (null, 'foo');
+  t.is (dec, null);
+});
+
 test ('do not fail verification if timeout unspecified', async (t) => {
   const obj = { foo: 'bar' };
   const str = crypto.sign_object (obj, 'baz');