This assert could be triggered during two phase commit if binary
log was used as transaction coordinator log. The triggered assert
checks that the same number of transaction IDs are processed in
the prepare and commit phases.

The reason it was triggered, was that the transaction consisted
of an INSERT/UPDATE IGNORE that had an ignorable error. Since it
had an error, no row log events were made and therefore
prepared_xids was 0. However, since it was an IGNORE statement,
the statement started a read/write statement transaction, committed
it and completed successfully.

This patch fixes the problem by adjusting the assert to take
this possibility into account.

Test case added to binlog.binlog_innodb_row.test.