Only display an ipv6 address if it exists

src/freedombone-controlpanel

     echo 'IP/DNS addresses'
     echo '================'
     echo ''
-    echo "IPv4: $(get_ipv4_address)    IPv6: $(get_ipv6_address)"
+    echo -n "IPv4: $(get_ipv4_address)"
+    ipv6_address="$(get_ipv6_address)"
+    if [ ${#ipv6_address} -gt 0 ]; then
+        echo "    IPv6: $(ipv6_address)"
+    fi
+    echo ''
     echo ''
 }