base-files: /lib/functions/network.sh: fix network_get_ipaddr6

When looking for the first ipaddr also consider the current prefix just
like network_get_ipaddrs6 does. If ipv6-address was empty the function
did not return the first ipaddr even if the list was non-empty.

fixes commit 83e9122f88a002871d5cdf421cf6aa6052b7e006

Signed-off-by: Henning Schild <henning@hennsch.de>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@42139 3c298f89-4303-0410-b956-a3cf2f4a3e73