Return empty when parsing a multi-part POST with only one end delimiter.

Fixed: rack#2103

Sending the following request in a browser generates a request with
with only one end delimiter.

```javascript
const formData = new FormData();
const request = new Request('http://127.0.0.1:8080/', {
  method: 'POST',
  body: formData,
});
const response = fetch(request);
```

```
curl 'http://127.0.0.1:8080/' \
  -H 'Content-Type: multipart/form-data; boundary=----WebKitFormBoundaryR1LC4tR6ayskIXJm' \
  --data-raw $'------WebKitFormBoundaryR1LC4tR6ayskIXJm--\r\n'
```

This request is not compliant RFC7578, but is generated by major browsers such as
FireFox and Chrome.
Supporting this request will cause the multipart parser to return an empty value.