MDEV-11154: Write_on_release_cache(log_event.cc) function will not write "COMMIT", if use "mysqlbinlog ... | mysql ..."

Problem:
=======
Executing command, "mysqlbinlog --read-from-remote-server --host='xx.xx.xx.xx'
--port=3306 --user=xxx --password=xxx --database=mysql --to-last-log
mysql-bin.000001 --start-position=1098699 --stop-never |mysql -uxxx -pxxx", we
found that last data read from remote couldn't commit.

Analysis:
========
The purpose of 'Write_on_release_cache' is that the contents of the Cache will
automatically be written to a dedicated result file on destruction. Flush
operation on the result file is controlled by a flag 'FLUSH_F'. Events which
require force flush upon their destruction will have to enable this
'Write_on_release_cache::FLUSH_F'. At present the 'FLUSH_F' flag is defined as
an enum as shown below.

enum flag
{
  FLUSH_F
};

Since 'FLUSH_F' is the first member without initialization it get the default
value '0'. Because of this the following flush condition never succeeds.

if (m_flags & FLUSH_F)
  fflush(m_file);

At present the file gets flushed only during my_fclose(result_file) operation.
When continuous streaming is enabled through --stop-never option it never gets
flushed and hence events are not replicated.

Fix:
===
Initialize the enum value to non zero value.