disconnect: set conn->data for protocol disconnect

Follow-up to fb445a1e18d: Set conn->data explicitly to point out the
current transfer when invoking the protocol-specific disconnect function
so that it can work correctly.

Bug: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=12173

lib/url.c

@@ -781,6 +781,9 @@ CURLcode Curl_disconnect(struct Curl_easy *data,
   Curl_http_ntlm_cleanup(conn);
 #endif
 
+  /* the protocol specific disconnect handler needs a transfer for its
+     connection! */
+  conn->data = data;
   if(conn->handler->disconnect)
     /* This is set if protocol-specific cleanups should be made */
     conn->handler->disconnect(conn, dead_connection);