The client program 'mysqlbinlog' crashed when trying to print a User_var_log_event holding
a floating-point value since the format specifier for my_b_printf() does not support
floating-point format specifiers.

This patch prints the floating-point number to an internal buffer, and then writes
that buffer to the output instead.


mysql-test/r/mysqlbinlog.result:
  Result file change.
mysql-test/t/mysqlbinlog.test:
  Adding test that mysqlbinlog can write and read back User_var_log_event
  for real, decimal, integer, and string. These are the only types supported
  for user variables.
sql/log_event.cc:
  Using my_sprintf() to print floating-point value of User_var_log_event value to a
  character buffer and then to the real output, since my_b_printf() does not
  support floating-point format. Also adding macro to give buffer size needed 
  for printing floating-point numbers in %g format.