[ticket/13743] Add $phpbb_root_path and $phpEx definitions in message_parser.php
message_parser.php performs include of bbcode.php at the very start using
$phpbb_root_path and $phpEx vars, so there's a possibility those can be
undefined at that point, especially when message_parser.php is being included
in event listener or another extension file where $phpbb_root_path and $phpEx
don't exist. This can be fixed by adding the appropriate vars definitions.
