redis-cli: fix prompt after shutdown command

Fix redis-cli prompt to state `not connected` after a `SHUTDOWN` command is sent.

Original scenario (before fix):

```
redis 127.0.0.1:6379> KEYS *
1) "aaa"
redis 127.0.0.1:6379> SHUTDOWN
redis 127.0.0.1:6379> KEYS *
redis 127.0.0.1:6379> GET aaa
redis 127.0.0.1:6379> 
```

Same scenario after redis-cli fix:

```
redis 127.0.0.1:6379> KEYS *
1) "aaa"
redis 127.0.0.1:6379> SHUTDOWN
not connected> KEYS *
Could not connect to Redis at 127.0.0.1:6379: Connection refused
not connected> GET aaa
Could not connect to Redis at 127.0.0.1:6379: Connection refused
not connected> 
```