Fix OOB read on empty hostnames in evdns.

Upstream bug report:

https://github.com/libevent/libevent/issues/332

Patch copied from upstream source repository:

https://github.com/libevent/libevent/commit/ec65c42052d95d2c23d1d837136d1cf1d9ecef9e

From a0305cec166a5bc89f1eb362510cc4cd25ecc0bc Mon Sep 17 00:00:00 2001
From: Azat Khuzhin <a3at.mail@gmail.com>
Date: Fri, 25 Mar 2016 00:33:47 +0300
Subject: [PATCH] evdns: fix searching empty hostnames

---
 evdns.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/evdns.c b/evdns.c
index 137c24ea..6191c677 100644
--- a/evdns.c
+++ b/evdns.c
@@ -3122,9 +3122,12 @@ search_set_from_hostname(struct evdns_base *base) {
 static char *
 search_make_new(const struct search_state *const state, int n, const char *const base_name) {
 	const size_t base_len = strlen(base_name);
-	const char need_to_append_dot = base_name[base_len - 1] == '.' ? 0 : 1;
+	char need_to_append_dot;
 	struct search_domain *dom;
 
+	if (!base_len) return NULL;
+	need_to_append_dot = base_name[base_len - 1] == '.' ? 0 : 1;
+
 	for (dom = state->head; dom; dom = dom->next) {
 		if (!n--) {
 			/* this is the postfix we want */
-- 
2.11.0

