Skip unnecessary mech calls in gss_inquire_cred()

If the caller does not request a name, lifetime, or cred_usage when
calling gss_inquire_cred(), service the call by copying the mechanism
list (if requested) but do not call into the mech.

This change alleviates an issue (reported by Adam Bernstein) where
SPNEGO can fail in the presence of expired krb5 credentials rather
than proceeding with a different mechanism, or can resolve a krb5
credential without the benefit of the target name.

(cherry picked from commit ff5eb89)

ticket: 8373
version_fixed: 1.14.2

src/lib/gssapi/mechglue/g_inq_cred.c

@@ -92,27 +92,32 @@ gss_OID_set *		mechanisms;
 	mech_cred = GSS_C_NO_CREDENTIAL;
 	mech = gssint_get_mechanism(GSS_C_NULL_OID);
     }
-    if (mech == NULL)
-	return (GSS_S_DEFECTIVE_CREDENTIAL);
-    if (!mech->gss_inquire_cred)
-	return (GSS_S_UNAVAILABLE);
 
-    status = mech->gss_inquire_cred(minor_status, mech_cred,
-				    name ? &mech_name : NULL,
-				    lifetime, cred_usage, NULL);
-    if (status != GSS_S_COMPLETE) {
-	map_error(minor_status, mech);
-	return(status);
-    }
+    /* Skip the call into the mech if the caller doesn't care about any of the
+     * values we would ask for. */
+    if (name != NULL || lifetime != NULL || cred_usage != NULL) {
+	if (mech == NULL)
+	    return (GSS_S_DEFECTIVE_CREDENTIAL);
+	if (!mech->gss_inquire_cred)
+	    return (GSS_S_UNAVAILABLE);
 
-    if (name) {
-	/* Convert mech_name into a union_name equivalent. */
-	status = gssint_convert_name_to_union_name(&temp_minor_status,
-						   mech, mech_name, name);
+	status = mech->gss_inquire_cred(minor_status, mech_cred,
+					name ? &mech_name : NULL,
+					lifetime, cred_usage, NULL);
 	if (status != GSS_S_COMPLETE) {
-	    *minor_status = temp_minor_status;
 	    map_error(minor_status, mech);
-	    return (status);
+	    return(status);
+	}
+
+	if (name) {
+	    /* Convert mech_name into a union_name equivalent. */
+	    status = gssint_convert_name_to_union_name(&temp_minor_status,
+						       mech, mech_name, name);
+	    if (status != GSS_S_COMPLETE) {
+		*minor_status = temp_minor_status;
+		map_error(minor_status, mech);
+		return (status);
+	    }
 	}
     }