mysql_real_escape_string incorrectly handles big5

#define TEST_BUG8378a_IN  "\xa1' + 10 -- "

#define TEST_BUG8378a_OUT "\\\xa1\\' + 10 -- "

/* set connection options */

struct my_option_st opt_bug8378a[] = {

  {MYSQL_SET_CHARSET_NAME, (char *) "big5"},

  {0, NULL}

};

int bug_8378a(MYSQL *mysql) {

  int rc, len;

  char out[128], buf[256];

  MYSQL_RES *res;

  MYSQL_ROW row;

  /* MXS-4898: MaxScale sends utf8mb4 in handshake OK packet */

  SKIP_MAXSCALE;

  len= mysql_real_escape_string(mysql, out, TEST_BUG8378a_IN, sizeof(TEST_BUG8378a_IN)-1);

  FAIL_IF(memcmp(out, TEST_BUG8378a_OUT, len), "wrong result");

  sprintf(buf, "SELECT '%s' FROM DUAL", out);

  rc= mysql_query(mysql, buf);

  check_mysql_rc(rc, mysql);

  if ((res= mysql_store_result(mysql))) {

    row= mysql_fetch_row(res);

    if (memcmp(row[0], TEST_BUG8378a_IN, sizeof(TEST_BUG8378a_IN)-1)) {

      mysql_free_result(res);

      return FAIL;

    }

    mysql_free_result(res);

  } else

    return FAIL;

  return OK;

}