ASSERTION THD->TRANSACTION.XID_STATE.XID.IS_NULL()
               FAILED

The triggered assert checks that the previous XA transaction has
done proper cleanup before a new XA transaction is started.
The bug that caused it to be triggered was that XA COMMIT did not
clean up error state if XA COMMIT discovered that the current XA
transaction had to be rolled back.

This patch fixes the problem by resetting the XA error state
before XA COMMIT calls ha_rollback_trans(). This allows following
XA transactions to be started without triggering the assert.

Test case added to xa.test.