gss: Empty application data messages must not trigger end-of-stream

Addresses an issue in which SSL application data messages with zero
payload are misinterpreted as end-of-stream. This bug causes JGlobus
to fail with more recent versions of curl.
Merge pull request #99 from gbehrmann/fix/v2.0/empty-ssl-message

gss: Empty application data messages must not trigger end-of-stream