Only get freezedata on found contests

We can't do a normal $contest==null check as that might leak information
if someone doesn't have access.

Author: vmcj

webapp/src/Controller/API/ContestController.php

@@ -364,7 +364,7 @@ public function problemsetAction(Request $request, string $cid): Response
 
         $hasAccess = $this->dj->checkrole('jury') ||
             $this->dj->checkrole('api_reader') ||
-            $contest->getFreezeData()->started();
+            $contest?->getFreezeData()->started();
 
         if (!$hasAccess) {
             throw new AccessDeniedHttpException();