Occasionally mysqlbinlog --hexdump failed with error:
  ERROR 1064 (42000) at line ...: You have an error in your
  SQL syntax; check the manual that corresponds to your MySQL
  server version for the right syntax to use near
  'Query thread_id=... exec_time=... error_code=...

When the length of hexadecimal dump of binlog header was
divisible by 16, commentary sign '#' after header was lost.
The Log_event::print_header function has been modified to always
finish hexadecimal binlog header with "\n# ".


sql/log_event.cc:
  Fixed bug #28293.
  The Log_event::print_header function has been modified to always
  finish hexadecimal binlog header with "\n# ".
mysql-test/r/mysqlbinlog.result:
  Updated test case for bug #28293.
mysql-test/t/mysqlbinlog.test:
  Updated test case for bug #28293.