[krbdev.mit.edu #3467] Allow GSS_C_NO_OID in krb5_gss_canon_name

[Public Submitter via RT]

Some programs (most notably racoon from ipsec-tools) call generic
functions that end up calling krb5_gss_canonicalize_name with
GSS_C_NO_OID as the mechanism.  Currently, this results in a segfault,
which is clearly incorrect.  The attached patch quietly assumes the krb5
mechanism, since that's the only non-deprecated mechanism provided by
the library.

Patch provided by Daniel Kahn Gillmor.