BUG/MINOR: resolvers: shut off the warning for the default resolvers

When the resolv.conf file is empty or there is no resolv.conf file, an
empty resolvers will be created, which emits a warning during the
postparsing step.

This patch fixes the problem by freeing the resolvers section if the
parsing failed or if the nameserver list is empty.

Must be backported in 2.6, the previous patch which introduces
resolvers_destroy() is also required.
diff --git a/src/resolvers.c b/src/resolvers.c
index ce08479..c7267aa 100644
--- a/src/resolvers.c
+++ b/src/resolvers.c
@@ -3671,13 +3671,31 @@
 {
 	int err_code = 0;
 
+	/* if the section already exists, do nothing */
 	if (find_resolvers_by_id("default"))
 		return 0;
 
+	curr_resolvers = NULL;
 	err_code |= resolvers_new(&curr_resolvers, "default", "<internal>", 0);
-	if (!(err_code & ERR_CODE))
-		err_code |= parse_resolve_conf(NULL, NULL);
+	if (err_code & ERR_CODE)
+		goto err;
+	err_code |= parse_resolve_conf(NULL, NULL);
+	if (err_code & ERR_CODE)
+		goto err;
+	/* check if there was any nameserver in the resolvconf file */
+	if (LIST_ISEMPTY(&curr_resolvers->nameservers)) {
+		err_code |= ERR_FATAL;
+		goto err;
+	}
+
+err:
+	if (err_code & ERR_CODE) {
+		resolvers_destroy(curr_resolvers);
+		curr_resolvers = NULL;
+	}
 
+	/* we never return an error there, we only try to create this section
+	 * if that's possible */
 	return 0;
 }