Completion patch, since it is possible that Log_event::write_header()
is called with no current thread.

When starting the server, there is no thread available, but
MYSQL_BIN_LOG::open() is called to initialize the binary log. The
function writes a format description log event to the binary log,
which means that Log_event::write_header() can be called with no
current thread object available.


sql/log_event.cc:
  Since Log_event::write_header() can be called from inside
  init_server_components(), it is possible that thd is NULL. In this
  case, we set the time of the event to the current time instead
  of the start time of the query.