Method* Method::checked_resolve_jmethod_id(jmethodID mid) {
  if (mid == NULL) return NULL;
  Method* o = resolve_jmethod_id(mid);
  if (o == NULL || o == JNIMethodBlock::_free_method || !((Metadata*)o)->is_method()) {
    return NULL;
  }
  return o;
};

We could, in addition, test that:
- Method* is a valid metaspace pointer
- Method* is a method

see Metaspace::contains() and Method::is_valid_method(), respectively.

Can be done either unconditionally, or at least as assert, if it costs too much time.