gluon-mesh-batman-adv: fix radvd start with dns.servers, but without next_node.ip config

package/gluon-mesh-batman-adv/luasrc/lib/gluon/radvd/arguments

@@ -1,6 +1,9 @@
 #!/usr/bin/lua
+
 local site = require "gluon.site_config"
+
 io.write("-i local-node -p " .. site.prefix6)
-if site.dns and site.dns.servers then
+
+if site.dns and site.dns.servers and site.next_node and site.next_node.ip6 then
 	io.write(" --rdnss " .. site.next_node.ip6)
 end