[native kerberos] setting channelBinding of gssContext when not used

Gs2SaslServer: gssContext's channelBinding setting leads to error when native Kerberos is used.

This lead to following error when using native Kerberos library:

[GSSLibStub_acceptContext] before2: pCred=35810112, pContext=0
[GSSLibStub_acceptContext] before3: inToken.length=515
[GSSLibStub_acceptContext] after: pCred=35810112, pContext=0, pDelegCred=0
[GSSLibStub_acceptContext] after2: major=262144, GSS_ERROR(major)=262144 minor=12
[GSSLibStub_acceptContext] acceptSecContext JK Status major/minor = 40000/12
c/r/s = 0/4/0

Which mean routine error 4 has occurred, which is GSS_S_BAD_BINDINGS - Incorrect channel bindings were supplied.

This is fixed when I change cb (in native) to GSS_C_NO_CHANNEL_BINDINGS - equivalent of setting null into channelBinding in gssContext.