telnet.c: fix OOM triggered segfault

lib/telnet.c

@@ -5,7 +5,7 @@
  *                            | (__| |_| |  _ <| |___
  *                             \___|\___/|_| \_\_____|
  *
- * Copyright (C) 1998 - 2011, Daniel Stenberg, <daniel@haxx.se>, et al.
+ * Copyright (C) 1998 - 2012, 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
@@ -1273,11 +1273,13 @@ static CURLcode telnet_done(struct connectdata *conn,
   (void)status; /* unused */
   (void)premature; /* not used */
 
+  if(!tn)
+    return CURLE_OK;
+
   curl_slist_free_all(tn->telnet_vars);
   tn->telnet_vars = NULL;
 
-  free(conn->data->state.proto.telnet);
-  conn->data->state.proto.telnet = NULL;
+  Curl_safefree(conn->data->state.proto.telnet);
 
   return CURLE_OK;
 }