This was pointed out by another compiler warning. The 'name' parameter
of inquire_cred_by_mech is not a gss_name_t (which is the type of
GSS_C_NO_NAME); it's a gss_name_t *, because it's an _output_
parameter. We're not telling the library that we aren't _passing_ a
name: we're telling it that we don't need it to _return_ us a name. So
the appropriate null pointer representation is just NULL.
(This was harmless apart from a compiler warning, because gss_name_t
is a pointer type in turn and GSS_C_NO_NAME expands to a null pointer
anyway. It was just a wrongly-typed null pointer.)