evdns: fix race condition in evdns_getaddrinfo()

evdns_getaddrinfo() starts two parallel requests for A and AAAA record.
But if request is created from thread different from dns_base's, request of A record is
started immediately and may result in calling free_getaddrinfo_request() from
evdns_getaddrinfo_gotresolve() because `other_req' doesn't exist yet.

After that, request of AAAA record starts and finishes, and evdns_getaddrinfo_gotresolve()
is called again for structure that is already freed.

This commits adds locking into evdns_getaddrinfo() function.

evdns.c

@@ -4637,6 +4637,7 @@ evdns_getaddrinfo(struct evdns_base *dns_base,
 	int err;
 	int port = 0;
 	int want_cname = 0;
+	int started = 0;
 
 	if (!dns_base) {
 		dns_base = current_base;
@@ -4715,6 +4716,8 @@ evdns_getaddrinfo(struct evdns_base *dns_base,
 	 * launching those requests. (XXX we don't do that yet.)
 	 */
 
+	EVDNS_LOCK(dns_base);
+
 	if (hints.ai_family != PF_INET6) {
 		log(EVDNS_LOG_DEBUG, "Sending request for %s on ipv4 as %p",
 		    nodename, &data->ipv4_request);
@@ -4741,7 +4744,11 @@ evdns_getaddrinfo(struct evdns_base *dns_base,
 	evtimer_assign(&data->timeout, dns_base->event_base,
 	    evdns_getaddrinfo_timeout_cb, data);
 
-	if (data->ipv4_request.r || data->ipv6_request.r) {
+	started = (data->ipv4_request.r || data->ipv6_request.r);
+
+	EVDNS_UNLOCK(dns_base);
+
+	if (started) {
 		return data;
 	} else {
 		mm_free(data);