Fix SASL negotiation on JRuby

On MRI, calling `read(0)` immediately returns an empty string. On JRuby, calling `read(0)` blocks forever. See issue https://github.com/jruby/jruby/issues/1637

Looking at the Java Thrift client, https://github.com/apache/thrift/blob/master/lib/java/src/org/apache/thrift/transport/TSaslTransport.java#L285  once negotiation status is complete we don't need to do anything else so this should be a safe change to make.