Fix SISMEMBER should return 0 when key does not exist (#1661)

Because we didn't check for not found, causing SISMEMBER
to return an error instead of 0 when the key doesn't exist:
```
127.0.0.1:6379> SISMEMBER foo bar
(integer) 0

127.0.0.1:6666> SISMEMBER foo bar
(error) ERR NotFound:
```

I also checked smismember and it's ok.

Fixes #1659.

src/commands/cmd_set.cc

@@ -104,7 +104,7 @@ class CommandSIsMember : public Commander {
     redis::Set set_db(svr->storage, conn->GetNamespace());
     bool ret = false;
     auto s = set_db.IsMember(args_[1], args_[2], &ret);
-    if (!s.ok()) {
+    if (!s.ok() && !s.IsNotFound()) {
       return {Status::RedisExecErr, s.ToString()};
     }