- Zmey Petroff found a crash when libcurl accessed a NULL pointer, which

happened if you set the connection cache size to 1 and for example failed to
  login to an FTP site. Bug report #1896698
  (http://curl.haxx.se/bug/view.cgi?id=1896698)

lib/transfer.c

@@ -2389,8 +2389,12 @@ CURLcode Curl_perform(struct SessionHandle *data)
         if(CURLE_OK == res)
           res = res2;
       }
-      else
-        /* Curl_do() failed, clean up left-overs in the done-call */
+      else if(conn)
+        /* Curl_do() failed, clean up left-overs in the done-call, but note
+           that at some cases the conn pointer is NULL when Curl_do() failed
+           and the connection cache is very small so only call Curl_done() if
+           conn is still "alive".
+        */
         res2 = Curl_done(&conn, res, FALSE);
 
       /*