XA START may cause assertion failure/server crash when it is called
after unilateral roll back issued by the Resource Manager (both
in regular transaction and after XA transaction).

The problem was that rm_error variable wasn't set/reset properly.

mysql-test/r/xa.result:
  A test case for BUG#43171.
mysql-test/t/xa.test:
  A test case for BUG#43171.
sql/handler.cc:
  Setting rm_error when we're out of XA transaction has no
  special meaning. But it blocks reset of thd->transaction.xid
  structure later.
sql/sql_parse.cc:
  Reset rm_error before we enter ha_rollback(), so
  thd->transaction.xid strucure is reinitialized.