In routerlist_assert_ok(), check r2 before taking &(r2->cache_info)

Technically, we're not allowed to take the address of a member can't
exist relative to the null pointer.  That makes me wonder how any sane
compliant system implements the offsetof macro, but let's let sleeping
balrogs lie.

Fixes 13096; patch on 0.1.1.9-alpha; patch from "teor", who was using
clang -fsanitize=undefined-trap -fsanitize-undefined-trap-on-error -ftrapv