Assert in Diagnostics_area::set_ok_status() for XA COMMIT

This assert was triggered if XA COMMIT was issued when an XA transaction
already had encountered an error (e.g. a deadlock) which required
the XA transaction to be rolled back.

In general, the assert is triggered if a statement tries to send OK to
the client when an error has already been reported. It was triggered
in this case because the trans_xa_commit() function first reported an
error, then rolled back the transaction and finally returned FALSE,
indicating success. Since trans_xa_commit() reported success,
mysql_execute_command() tried to report OK, triggering the assert.

This patch fixes the problem by fixing trans_xa_commit() to return TRUE
if it encounters an error that requires rollback, even if the rollback
itself is successful.

Test case added to xa.test.