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

We must not dispose() of any credential handle passed to the
NativeGSSContext() constructor. But we must dispose() of credentials
that are acquired in NativeGSSContext.

This is very important because the JVM does not know about the size of
the JNI credential objects, so it doesn't readily recognize memory
pressure from them, leading to memory pressure issues in SASL and GSS
server applications.