MAPG-235 challenge token related error handling

src/Repository/UserInChallengeRepository.php

@@ -48,6 +48,13 @@ class UserInChallengeRepository
             $withRelations = array_unique(array_merge($withRelations, $necessaryRelations));
         }
 
+        // 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);
 
         $select = new Select(\Container::$dbConnection);