Validate X-Forwarded-For attacks to prevent error, also could add attackers to firewall

I get a lot of attacks using the X-Forwarded-For header, i.e.:
`
}__test|O:21:"JDatabaseDriverMysqli":3:{s:2:"fc";O:17:"JSimplepieFactory":0:{}s:21:"\0\0\0disconnectHandlers";a:1:{i:0;a:2:{i:0;O:9:"SimplePie":5:{s:8:"sanitize";O:20:"JDatabaseDriverMysql":0:{}s:8:"feed_url";s:56:"die(md5(DIRECTORY_SEPARATOR));JFactory::getConfig();exit";s:19:"cache_name_function";s:6:"assert";s:5:"cache";b:1;s:11:"cache_class";O:20:"JDatabaseDriverMysql":0:{}}i:1;s:4:"init";}}s:13:"\0\0\0connection";b:1;}�
`

This produces a 500 error in AttackBlocker.php on line 450:
`gethostbyaddr(): Address is not a valid IPv4 or IPv6 address`

Can we handle the error to avoid the 500? Also, could we add an option to firewall these attackers? Although, given the fake X-Forwarded-For we would have to use a different method of determining their IP.

Start at AttackBlocker.php line 450 and inspect the gethostbyaddr() call that receives the malformed X-Forwarded-For value. Reproduce the request to confirm the 500 error, then determine how invalid forwarded addresses should be handled and how attacker blocking should identify a trustworthy IP. Done means the malformed header no longer causes a 500 and the intended firewall behavior is covered by the project’s tests, if available.

Written by the indexing model from the issue text.