Curl_disconnect: treat all CONNECT_ONLY connections as "dead"

Since the connection has been used by the "outside" we don't know the state of it anymore and curl should not use it anymore. Bug: https://curl.haxx.se/mail/lib-2019-04/0052.html Closes #3795

Author: bagder

lib/url.c

@@ -809,6 +809,10 @@ CURLcode Curl_disconnect(struct Curl_easy *data,
  for the connection! */
  conn->data = data;
 
+ if(conn->bits.connect_only)
+ /* treat the connection as dead in CONNECT_ONLY situations */
+ dead_connection = TRUE;
+
  if(conn->handler->disconnect)
  /* This is set if protocol-specific cleanups should be made */
  conn->handler->disconnect(conn, dead_connection);