Instead of throwing an assertion, fail DHCPv6 when a IPv6 link-local
address is not configured on the device. There are different reasons
why the assertion may fail: for example the address was removed
externally; or the device is gone (and thus the platform already
received the notification of addresses removal) but the device is still
connecting because its disposal happens in an idle callback.

None of these deserves an assertion, which should only be for
programming errors.

https://bugzilla.redhat.com/show_bug.cgi?id=1432251