fixed uninit memory access in SET pluginvar=DEFAULT
innodb_mysql.test, innodb_mysql.result:
  test case for SET pluginvar=DEFAULT


mysql-test/r/innodb_mysql.result:
  test case for SET pluginvar=DEFAULT
mysql-test/t/innodb_mysql.test:
  test case for SET pluginvar=DEFAULT
sql/sql_plugin.cc:
  fixed uninit memory access in SET pluginvar=DEFAULT