Requesting /files/read.json with a negative length value causes subsequent /files requests to 404.

I accidentally hit a slave's /files/read.json endpoint with a negative length (ex. http://hostname:5051/files/read.json?path=XXX&offset=0&length=-100). The HTTP request timed out after 30 seconds with nothing relevant in the slave logs, and subsequent calls to any of the /files endpoints on that slave immediately returned a HTTP 404 response. We ultimately got things working again by restarting the mesos-slave process (checkpointing FTW!), but it'd be wise to guard against negative lengths on the slave's end too.