when it was printing a Query event, it produced invalid SQL (missing
the BINLOG keyword, so the SQL started with the base64 string, which
is incorrect).
Note: no testcase; I have a .test which shows that the bugfix works,
but it triggers BUG#26361 and so gives Valgrind warnings. I'm sending
this test to the fixer of BUG#26361 for her/him to push when she/he
fixes BUG#26361.


client/mysqlbinlog.cc:
  writing the header (a line started with "#", i.e. a comment) and the
  body (the real operation) of an event to the same IO_CACHE
  (result_cache) confused the logic of Log_event::print_base64()
  (which is that if the cache is not empty then the BINLOG keyword
  should not be printed); it caused the BINLOG keyword to miss hence
  a syntactically wrong output of "mysqlbinlog --base64-output"
  for Query events.
  So we just use the two IO_CACHE already available in "print_event_info".
sql/log_event.cc:
  using the new small inline function.
  Note that the replication code should one day be fixed to trap all
  errors (like disk write errors).
sql/log_event.h:
  small inline function to group two operations: copying an IO_CACHE
  to a FILE, and reinitializing this IO_CACHE for being filled again.
sql/records.cc:
  fix after merge