assert due to sending status to client two times

[Seppo Jaakola]

Following assert happened with sysbench testing:

mysqld: protocol.cc:386: void net_end_statement(THD*): Assertion `! thd->main_da.is_sent' failed.

This happens, when committing local trx was aborted due to lock conflict with slave applier, but
certification of the local trx was successful. Reason for this is asymmetric lock granularity between wsrep
and innodb.

Wsrep performs at this point transaction replaying in local state, to make sure each node will have same
verdict of the certification. Apparently this replay logic can sometimes lead to sending status to client
two times.

This assert has not been diagnosed with 5.1.30 build. Possibly it is due to 5.1.31 or 5.1.32 merging.