mqtt: on send error, return error

Reported-by: Maciej Domanski

Closes #10623

lib/mqtt.c

@@ -122,8 +122,9 @@ static CURLcode mqtt_send(struct Curl_easy *data,
   struct MQTT *mq = data->req.p.mqtt;
   ssize_t n;
   result = Curl_write(data, sockfd, buf, len, &n);
-  if(!result)
-    Curl_debug(data, CURLINFO_HEADER_OUT, buf, (size_t)n);
+  if(result)
+    return result;
+  Curl_debug(data, CURLINFO_HEADER_OUT, buf, (size_t)n);
   if(len != (size_t)n) {
     size_t nsend = len - n;
     char *sendleftovers = Curl_memdup(&buf[n], nsend);