Protect against baseURLs ending with a '/'

Author: vmcj

webapp/src/Service/ExternalContestSourceService.php

@@ -1454,6 +1454,7 @@ protected function importSubmission(Event $event, EventData $data): void
                     // Relative URL, prepend the base URL.
                     $zipUrl = ($this->basePath ?? '') . $zipUrl;
                 }
+                $zipUrl = str_replace('//', '/', $zipUrl);
 
                 $tmpdir = $this->dj->getDomjudgeTmpDir();