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.
(cherry picked from commit 4fcd4f1e0569b5952ecb8ab097ea5bf51c9d2452)
Merge pull request #101 from gbehrmann/fix/empty-ssl-message

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