GSSAPI fix: don't pass GSS_C_NO_NAME to inquire_cred_by_mech.

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.)

ssh/gssc.c

@@ -75,7 +75,7 @@ static Ssh_gss_stat ssh_gssapi_acquire_cred(struct ssh_gss_library *lib,
         gssctx->maj_stat =
             gss->inquire_cred_by_mech(&gssctx->min_stat, cred,
                                       (gss_OID) GSS_MECH_KRB5,
-                                      GSS_C_NO_NAME,
+                                      NULL,
                                       &time_rec,
                                       NULL,
                                       NULL);