curl_slist_append.3: add error checking to the example

.\" *                            | (__| |_| |  _ <| |___

.\" *                             \___|\___/|_| \_\_____|

.\" *

.\" * Copyright (C) 1998 - 2011, Daniel Stenberg, <daniel@haxx.se>, et al.

.\" * Copyright (C) 1998 - 2015, Daniel Stenberg, <daniel@haxx.se>, et al.

.\" *

.\" * This software is licensed as described in the file COPYING, which

.\" * you should have received as part of this distribution. The terms

 struct curl_slist *slist=NULL;

 slist = curl_slist_append(slist, "pragma:");

 if (slist == NULL)

   return -1;

 curl_easy_setopt(handle, CURLOPT_HTTPHEADER, slist);

 curl_easy_perform(handle);