An INSERT query log event is preceeded by an INSERT_ID intvar event if the
INSERT allocates a new auto_increment value. But if we ignore the INSERT
due to --replicate-ignore-table or similar, then the INSERT_ID event is
still executed, and the set value of INSERT_ID lingers around in the
slave sql thread THD object indefinitely until the next INSERT that
happens to need allocation of a new auto_increment value.

Normally this does not cause problems as such following INSERT would
normally come with its own INSERT_ID event. In this bug, the user had
a trigger on the slave which was missing on the master, and this
trigger had an INSERT which could be affected. In any case, it seems
better to not leave a stray INSERT_ID hanging around in the sql thread
THD indefinitely.

Note that events can also be skipped from apply_event_and_update_pos();
however it is not possible in that code to skip the INSERT without also
skipping the INSERT_ID event.