Bug report #1759542 (http://curl.haxx.se/bug/view.cgi?id=1759542). A bad use

of a socket after it has been closed, when the FTP-SSL data connection is taken
down.

lib/ftp.c

@@ -3115,6 +3115,14 @@ CURLcode Curl_ftp_done(struct connectdata *conn, CURLcode status, bool premature
   shutdown(conn->sock[SECONDARYSOCKET],2);  /* SD_BOTH */
 #endif
 
+  if(conn->ssl[SECONDARYSOCKET].use) {
+    /* The secondary socket is using SSL so we must close down that part first
+       before we close the socket for real */
+    Curl_ssl_close(conn, SECONDARYSOCKET);
+
+    /* Note that we keep "use" set to TRUE since that (next) connection is
+       still requested to use SSL */
+  }
   sclose(conn->sock[SECONDARYSOCKET]);
 
   conn->sock[SECONDARYSOCKET] = CURL_SOCKET_BAD;