Do not skip e-mail validation if $g_login_method != LDAP

Prior to this, function email_is_valid() always returned true when
$g_use_ldap_email = ON, even if $g_login_method != LDAP.

Even though this is not an usual case (when not using LDAP, the other
LDAP-related configuration options should be left to their default
values), this causes the actual validation to be skipped which could
lead to email sending errors.

Fixes #14632

core/email_api.php

@@ -113,7 +113,7 @@ function email_is_valid( $p_email ) {
 		return true;
 	}
 
-	if ( ON == config_get( 'use_ldap_email' ) ) {
+	if ( LDAP == config_get( 'login_method' ) && ON == config_get( 'use_ldap_email' ) ) {
 		return true;
 	}