ftplistparser: keep state between invokes

Regression from f786d1f14

Reported-by: wncboy on github
Fixes #2445
Closes #2508

lib/ftplistparser.c

@@ -5,7 +5,7 @@
  *                            | (__| |_| |  _ <| |___
  *                             \___|\___/|_| \_\_____|
  *
- * Copyright (C) 1998 - 2017, Daniel Stenberg, <daniel@haxx.se>, et al.
+ * Copyright (C) 1998 - 2018, 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
@@ -1003,6 +1003,7 @@ size_t Curl_ftp_parselist(char *buffer, size_t size, size_t nmemb,
 
     i++;
   }
+  return retsize;
 
 fail: