vio_end() is called twice in this fragment in libmysql/libmysql.c in case of embedded server:

void STDCALL mysql_server_end()

{

  if (!mysql_client_init)

    return;

  mysql_client_plugin_deinit();

#ifdef EMBEDDED_LIBRARY

  end_embedded_server();

#endif

  finish_client_errs();

  vio_end();

First time it's called from end_embedded_server() via clean_up().
Second time it's called from mysql_server_end() directly.

The problem does not show up in any ways with the standard
yassl and openssl libraries, but it's reported that mysqltest_embedded
hangs when linked against a custom built openssl library.
See the comments in the end of MDEV-4890.