MAPG-235 challenge token related error handling

src/Repository/ChallengeRepository.php

@@ -31,6 +31,13 @@ class ChallengeRepository
 
     public function getByTokenStr(string $token_str): ?Challenge
     {
+        // validate token string
+        foreach (str_split($token_str) as $char) {
+            if (!(('0' <= $char && $char <= '9') || ('a' <= $char && $char <= 'f'))) {
+                return null;
+            }
+        }
+        // convert token to int
         $token = hexdec($token_str);
 
         return $this->getByToken($token);