Problem: when the server reads a log_event from file, it should read
the post-header lengths from the format_description_log_event. Some
event types which currently have post-header length 0 did not do this,
and instead had a hard-coded zero length for the post-header. That
means the current server version will not be able to read future
versions of these events.
Fix: make the reader functions read the post-header.


sql/log_event.cc:
   - Made Format_description_log_event constructor initialize all
     post-header lengths explicitly, to make it easier to find them
     in the source code.
   - After this, it is no longer necessary to pass the MY_ZEROFILL
     flag to my_malloc. I removed the flag and added a sanity-check
     that will be executed only in debug-mode.
   - Made INTVAR, RAND, USER_VAR, and XID events skip post_header_len
     when reading from file.
sql/log_event.h:
  Added explicit defines for the lengths of all event types.