Fix validateLock being called more than once

Node crashes if locked file is locked again.

```
Error: Can't render headers after they are sent to the client.
```

How to reproduce:

```
node examples/fileserver.js
curl localhost:8000/1.txt -X LOCK -H "Content-type: text/xml" -d '<?xml version="1.0"?><A:lockinfo xmlns:A="DAV:"><A:locktype><A:write/></A:locktype><A:lockscope><A:shared/></A:lockscope><A:owner><A:href>CURL</A:href></A:owner></A:lockinfo>'
curl localhost:8000/1.txt -X LOCK -H "Content-type: text/xml" -d '<?xml version="1.0"?><A:lockinfo xmlns:A="DAV:"><A:locktype><A:write/></A:locktype><A:lockscope><A:shared/></A:lockscope><A:owner><A:href>CURL</A:href></A:owner></A:lockinfo>'
```