RFR 8212217: JGSS: Don't dispose() of creds too eagerly

[Weijun Wang]

Please take a review at

   http://cr.openjdk.java.net/~weijun/8212217/webrev.00/

This bug is reported and fixed by Nico Williams <nico at twosigma.com>.

I'll think if a regression test can be added.

Thanks
Max