reset cached client plugin when it's no longer needed

it's a pointer into the net buffer, so it might be overwritten by the
next read or write. And the next plugin switch (in multi-auth) will
try to compare it (in send_plugin_request_packet) which is normally
harmless but fails the assert with Lex_ident::is_valid_ident()

sql/sql_acl.cc

@@ -14230,6 +14230,7 @@ static int server_mpvio_write_packet(MYSQL_PLUGIN_VIO *param,
  res= my_net_write(&mpvio->auth_info.thd->net, packet, packet_len) ||
  net_flush(&mpvio->auth_info.thd->net);
  }
+ mpvio->cached_client_reply.plugin= ""_LEX_CSTRING;
  mpvio->status= MPVIO_EXT::FAILURE; // the status is no longer RESTART
  mpvio->packets_written++;
  DBUG_RETURN(res);