[ticket/10240] Added code to strip control characters from messages.

It is possible to evade the word filter using control characters.

Seeing as control characters have no practical application in a bulletin
board software, it doesn't do any harm to remove them entirely. That is
what this commit does.

PHPBB3-10240

phpBB/includes/functions_content.php

@@ -691,6 +691,9 @@ function censor_text($text)
 		return '';
 	}
 
+	// Strip control characters
+	$text = preg_replace('/[\x00-\x0f]/', '', $text);
+
 	// We moved the word censor checks in here because we call this function quite often - and then only need to do the check once
 	if (!isset($censors) || !is_array($censors))
 	{