set booking user to current user if resource booking is changed or if the user...

set booking user to current user if resource booking is changed or if the user is invalid, fixes #1220

Closes #1220

Merge request studip/studip!729

lib/models/resources/ResourceBooking.class.php

@@ -466,8 +466,10 @@ class ResourceBooking extends SimpleORMap implements PrivacyObject, Studip\Calen
         }
 
         // update the booking user
-        $this->booking_user = User::findCurrent();
-    
+        if (!$this->isNew() || !$this->booking_user_id) {
+            $this->booking_user = User::findCurrent();
+        }
+
         //Check if the user has booking rights on the resource.
         //The user must have either permanent permissions or they have to
         //have booking rights by a temporary permission in this moment