When statement-based replication is used, and the
transaction isolation level is READ-COMMITTED or stricter,
InnoDB will print an error because statement-based
replication might lead to inconsistency between master
and slave databases. However, when the binary log is not
engaged, this is not an issue and an error should
not be printed.

This patch makes thd_binlog_format() return BINLOG_FORMAT_
UNSPEC when the binary log is not engaged for the given
thread.