Fix for NPE when the OpenSSLSocketImpl returns null from getNpnSelectedProtocol

I [detailed this issue here](https://gist.github.com/adamsp/7ffd33906abf9b1a5df3) when trying to track down a different issue (which I'm working through with Jesse [on StackOverflow](http://stackoverflow.com/questions/20306498/unable-to-close-chunkedinputstream-quickly-on-android-4-4-kitkat)).