curl_slist_append(): fix error detection
diff --git a/lib/slist.c b/lib/slist.c
index 123ea05..108d0f0 100644
--- a/lib/slist.c
+++ b/lib/slist.c
@@ -90,7 +90,7 @@
 {
   char *dupdata = strdup(data);
 
-  if(!data)
+  if(!dupdata)
     return NULL;
 
   list = Curl_slist_append_nodup(list, dupdata);