nsqd: malicious client can (easily) crash server

nsqd can be easily crashed. Steps to reproduce:

* nsq and run nsqd (using the default settings)
* Telnet to the port it is running on
* Type '  V2IDENTIFY', press enter and then type 'ctrl-c'
* The server will panic.
    panic: runtime error: makeslice: len out of range

I am running v0.2.27 on OS X if that make any difference.