Fix for BUG#16559 "Replication Problems with Non transactional tables inside an interrupted trans.":
problem was: when a connection disconnects having an open transaction affecting MyISAM and InnoDB, the ROLLBACK event stored in the binary log
contained a non-zero error code (1053 because of the disconnection), so when slave applied the transaction, slave complained that its ROLLBACK succeeded
(error_code=0) while master's had 1053, so slave stopped. But internally generated binlog events such as this ROLLBACK
should always have 0 as error code, as is true in 4.1 and was accidentally broken in 5.0,
so that there is no false alarm.