webif: Fix IPv4 fallback when IPv6 is not available.

If IPv6 support was enabled but available the WebIf failed to fallback
on using IPv4.

This fixes ticket #3404.

git-svn-id: http://streamboard.de.vu/svn/oscam/trunk@8834 4b0bc96b-bc66-0410-9d44-ebda105a78c1