If IPv6 prefix assignment is disabled, the "local-address" structure
might exist, but be empty which causes the adress formatting in the
network model class to bail out.
Verify the completeness of the "local-address" structure before using
it in order to avoid runtime errors.
Fixes#1657.
Signed-off-by: Jo-Philipp Wich <jo@mein.io>
bf49505ea0