restrict webauthn-tfa-get-args sql query

data/web/json_api.php

@@ -452,8 +452,11 @@ if (isset($_GET['query'])) {
           }
         break;
         case "webauthn-tfa-get-args":
-          $stmt = $pdo->prepare("SELECT `keyHandle` FROM `tfa` WHERE username = :username AND authmech = `webauthn`");
-          $stmt->execute(array(':username' => $_SESSION['pending_mailcow_cc_username']));
+          $stmt = $pdo->prepare("SELECT `keyHandle` FROM `tfa` WHERE username = :username AND authmech = :authmech");
+          $stmt->execute(array(
+            ':username' => $_SESSION['pending_mailcow_cc_username'],
+            ':authmech' => 'webauthn'
+          ));
           $rows = $stmt->fetchAll(PDO::FETCH_ASSOC);
           if (count($rows) == 0) {
             print(json_encode(array(