[ticket/12494] Undefined index: user_type on viewtopic.php

PHPBB3-12494
Merge pull request #2405 from Nicofuma/ticket/12494

[ticket/12494] Undefined index: user_type on viewtopic.php

* Nicofuma/ticket/12494:
  [ticket/12494] Undefined index: user_type on viewtopic.php