Some cpe routers announce invalidated ipv6 prefixes with a preflt and validlt of 0
for some time after they get a new prefix. the default routes for these prefixes are
getting continually removed and added with every ra we receive, so just don't bother.

At least my fritzbox behaves that way. rfc 6204 states that in this case preflt should be 0
and validlt should be 2 hours or it's current value and then counting down. But here we are.