tighten if-else and add empty skylink protection

docker/nginx/conf.d/client.conf

@@ -159,9 +159,7 @@ server {
 				if skylink == nil then
 					skylink, skylink_rest = string.match(hnsres_json.skylink, "/?([^/?]+)(.*)")
 				end
-			end
-
-			if hnsres_json.registry then
+			elseif hnsres_json.registry then
 				local publickey = hnsres_json.registry.publickey
 				local datakey = hnsres_json.registry.datakey
 
@@ -183,6 +181,11 @@ server {
 				skylink = data
 			end
 
+			-- fail with a generic 404 if skylink has not been extracted from a valid /hnsres response for some reason
+			if not skylink then
+				ngx.exit(ngx.HTTP_NOT_FOUND)
+			end
+
 			ngx.var.skylink = skylink
 			if request_uri_rest == "/" and skylink_rest ~= "" and skylink_rest ~= "/" then
 				ngx.var.rest = skylink_rest