Avoid segfault on weird timeout during name lookup.

If an evdns_getaddrinfo timeout happens while pending_cb is set, and
a callback is about to run, but we get a call to
evdns_getaddrinfo_gotresolve before it finishes.

Github issue libevent#60. Thanks to Greg Hazel for patch and patience.

evdns.c

@@ -4214,13 +4214,17 @@ evdns_getaddrinfo_timeout_cb(evutil_socket_t fd, short what, void *ptr)
 
 	/* Cancel any pending requests, and note which one */
 	if (data->ipv4_request.r) {
+		/* XXXX This does nothing if the request's callback is already
+		 * running (pending_cb is set). */
 		evdns_cancel_request(NULL, data->ipv4_request.r);
 		v4_timedout = 1;
 		EVDNS_LOCK(data->evdns_base);
 		++data->evdns_base->getaddrinfo_ipv4_timeouts;
 		EVDNS_UNLOCK(data->evdns_base);
 	}
 	if (data->ipv6_request.r) {
+		/* XXXX This does nothing if the request's callback is already
+		 * running (pending_cb is set). */
 		evdns_cancel_request(NULL, data->ipv6_request.r);
 		v6_timedout = 1;
 		EVDNS_LOCK(data->evdns_base);
@@ -4244,6 +4248,10 @@ evdns_getaddrinfo_timeout_cb(evutil_socket_t fd, short what, void *ptr)
 		data->user_cb(e, NULL, data->user_data);
 	}
 
+	data->user_cb = NULL; /* prevent double-call if evdns callbacks are
+			       * in-progress. XXXX It would be better if this
+			       * weren't necessary. */
+
 	if (!v4_timedout && !v6_timedout) {
 		/* should be impossible? XXXX */
 		free_getaddrinfo_request(data);
@@ -4314,6 +4322,13 @@ evdns_getaddrinfo_gotresolve(int result, char type, int count,
 		return;
 	}
 
+	if (data->user_cb == NULL) {
+		/* We already answered.  XXXX This shouldn't be needed; see
+		 * comments in evdns_getaddrinfo_timeout_cb */
+		free_getaddrinfo_request(data);
+		return;
+	}
+
 	if (result == DNS_ERR_NONE) {
 		if (count == 0)
 			err = EVUTIL_EAI_NODATA;