correct handling of dns fauilure

In case of domain name resolution error result can be negative (see Dns.cpp:46)

src/PubSubClient.cpp

@@ -122,7 +122,7 @@ boolean PubSubClient::connect(const char *id, const char *user, const char *pass
         } else {
             result = _client->connect(this->ip, this->port);
         }
-        if (result) {
+        if (result == 1) {
             nextMsgId = 1;
             // Leave room in the buffer for header and variable length field
             uint16_t length = 5;