fix undefined variable

Author: simonLeary42

resources/lib/UnityLDAP.php

@@ -91,7 +91,8 @@ public function getNextUIDGIDNumber($uid)
         $IDNumsInUse = array_merge($this->getAllUIDNumbersInUse(), $this->getAllGIDNumbersInUse());
         $start = $this->offset_UIDGID;
         $customIDMappings = $this->getCustomIDMappings();
-        if (array_key_exists($uid, $customIDMappings)) {
+        $customMappedID = $customIDMappings[$uid] ?? null;
+        if (!is_null($customMappedID) && !in_array($customMappedID, $IDNumsInUse)) {
             return $customMappedID;
         }
         if (!is_null($customMappedID) && in_array($customMappedID, $IDNumsInUse)) {