Incorrect handling of the interrupted AUTH command

Example server logs:

```
...
AUTH PLAIN *masked*
454 4.7.0 auth failed
* -> produces 'Command too short: "*"'
501 5.5.2 Bad command
QUIT
221 2.0.0 Bye
````

Expected behavior:

```
...
AUTH PLAIN *masked*
454 4.7.0 auth failed
*
501 5.0.0 Negotiation cancelled
QUIT
221 2.0.0 Bye
```

See related code:

client: https://github.com/emersion/go-smtp/blob/608f3c2840584931f2867716078c43485ea7ef3f/client.go#L353

server code which is triggered: https://github.com/emersion/go-smtp/blob/608f3c2840584931f2867716078c43485ea7ef3f/parse.go#L18
via https://github.com/emersion/go-smtp/blob/608f3c2840584931f2867716078c43485ea7ef3f/server.go#L175-L181

server code, which expected be triggered: https://github.com/emersion/go-smtp/blob/608f3c2840584931f2867716078c43485ea7ef3f/conn.go#L568-L572

Trace the interrupted AUTH flow from client.go around line 353 through server.go lines 175-181 and parse.go line 18, then compare it with the expected handling in conn.go lines 568-572. Reproduce the logged exchange and verify that a cancelled AUTH negotiation returns the expected 501 response instead of treating the input as a normal command.

Written by the indexing model from the issue text.