[mod_extforward] fix compilation without IPv6, (not) using undefined var (fixes #2515, thx mm)

From: Stefan Bühler <stbuehler@web.de>

git-svn-id: svn://svn.lighttpd.net/lighttpd/branches/lighttpd-1.4.x@2911 152afb58-edef-0310-8abb-c4023f1b3aa9