fix a possible crash in payrexx integration

src/server/backend/plugins/payments/payrexx/payrexxRoutes.js

@@ -133,8 +133,13 @@ module.exports = ( app, settings ) => {
     } );
 
     app.post( '/payments/webhook', bodyParser.json(), async ( req, res ) => {
+        if ( !req.body ) {
+            if ( !req.body.transaction ) {
+                res.status( 400 ).send( 'ERR_REQ_WRONG' );
+                return;
+            }
+        }
         if ( req.body.transaction.status === 'confirmed' ) {
-            console.error( gatewayReference[ req.body.transaction.referenceId ] );
             const response = await payrexx.getGateway( gatewayReference[ req.body.transaction.referenceId ] );
 
             if ( response.status === 200 ) {