[krbdev.mit.edu #1445] GSSAPI can fail to generate error in GSS_C_NO_CREDENTIAL case

[Sam Hartman via RT]

Nico points out that in accept_sec_context, cred->princ is used as the
server component of the call to krb5_mk_error.


This is problematic because sname and srealm are required fields and
cred->princ can be null in the gss_c_no_credential case.


I believe that if cred->princ is null you can get the principal out of
the decoded ap_req.